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BOOK 570. M52 v.2 c. 1 



3 T153 D01373fiE 

General and Professional Biology 


with Special Reference to Man 


Director of the Department of Zoology, Marquette University 
Late Professor of Biology, University of Dallas 







Second Edition 

Copyright 1922-1925 
Edward J. Menge 

Printed in the United States of America 

Introductory Embryology 

(Chick, Frog, and Mammal) 

Digitized by the Internet Archive 
in 2013 * 





Embryology of the Chick 11-118 


LAID — The Egg — The Reproductive Organs of the Fowl — Oogenesis 
— The Morula Stage — Blastulation— Gastrulation 11-30 


The Notochord — The Neural Plate — Metameric Division — Method of 
Illustrating Extra-Embryonic Portions 31-40 


THE FOUR TO SIX SOMITE STAGE (About Twenty-four Hours).... 41-45 


THE FIRST HALF OF THE SECOND DAY (Twenty-four to Thirty- 
six Hours) — The Differentiation of the Brain Region — Lengthening of 
the Fore-gut 46-51 


eight Hours) — The Brain — Torsion — The Circulatory System — The 
Path of a Blood-Corpuscle — The Excretory System 52-60 



and the Serosa — The Allantois — The Chorion 61-64 


Optic Vesicles — The Digestive Tract — The Lungs — The Liver — The 
Pancreas — The Thyroid Gland — The Thymus Gland — The Visceral 
Clefts and Visceral Arches — The Circulatory System 65-77 


tem 78-83 


tem — The Ganglia of the Cranial Nerves — The Spinal Cord — The 
Organs of Special Sense— The Eye— The Ear— The Nose— The Skeletal 
Structure — The Excretory System — The Reproductive System — The 
Adrenal Bodies — The Circulatory System — The Vitelline Circulation — 
The Allantoic Circulation — The Intra-Embryonic Circulation — The 
Heart — The Veins 84-111 



8 Table of Contents— Continued 


ment of the Skull— The Skull Proper— The Visceral Skull— The Heart. 114-118 

The Embryology of the Frog 119-194 


PARED WITH THAT OF THE CHICK— Classification of Chor- 
data — Embryology of the Frog — Fertilization — Maturation — The 
Formation of the Blastula — The Formation of the Gastrula — The 
Medullary Plate — The Formation of the Embryo — The Somites — The 
Later Development of the Tadpole — The Nervous System — The Fore- 
brain — The Mid-brain — The Hind-brain — The Peripheral Nervous Sys- 
tem — The Trigeminal or V-Nerve — The Facial and Auditory, or the 
VII and VIII Nerves — The Glossopharyngeal and Vagus (Pneumo- 
gastric) or IX and X Nerves — The Spinal Nerves — The Sympathetic 
System — The Eye — The Ear — The Nose — The Sense Organs of the 
Lateral Line 119-160 


THE DIGESTIVE TRACT— The Derivatives of the Mid-Gut— The 

Derivatives of the Hind-Gut 161-166 


THE MESODERMAL SOMITES— Table of Somites, Vertebrae, and 

Related Nerves of the Tadpole 167-169 


THE CIRCULAT9RY SYSTEM— The Heart— The Arterial System- 
Origin of the Circulatory System and the Blood — The Venous System 
— The Lymphatic System — The Septum Transversum 170-180 


THE UROGENITAL SYSTEM— The Mesonephros or Wolffian Body— 

The Reproductive System — The Adrenal Bodies or Epinephroi 181-187 


THE SKELETAL SYSTEM— The Skull 188-194 

Mammalian Embryology 195-209 


MAMMALIAN EMBRYOLOGY — Fertilization — The Blastoderm- 
Attachment of the Blastodermic Vesicle to the Uterine Wall — Implan- 
tation — The Embryonic Membranes — The Placenta — The Yolk-Sac — 
The Allantois — The Decidual Membranes — The Umbilical Cord 195-209 


Comparative Anatomy 211-466 





MAMMALS 215-239 

Tabi.e of Contents — Concluded 9 


THE INTEGUMENT — Fishes — Amphibia — Reptiles — Birds — 

Mammals — Hair — Glands — Scales 240-256 


THE ENDOSKELETON— The Vertebral Column— Regions of the Verte- 
bral Column — The Skull — The Appendicular Skeleton — Paired 
Appendages — The Shoulder Girdle — The Hip Girdle — The Free 
Appendages — The Limbs — Summary of the Cranium — Cyclostomata — 
Dogfish — Pisces — Amphibia — Aves • — Reptilia — Mammalia — 
Summary of the Skeletal System — The Dogfish — Amphibia — Reptilia — 
Aves — Mammalia 257-297 


THE DIGESTIVE SYSTEM— Detail Study— Teeth— Dental Formula- 
Epidermal Teeth — The Tongue — Glands — The Pharynx — The Oesopha- 
gus — The Stomach — The Intestine — The Liver — The Pancreas — 
Summary of the Digestive System — Amphioxus — Ascidians (Tunicates) 

— Fishes — Turtles — Aves — Mammals 298-332 


THE RESPIRATORY SYSTEM— Amphibia— The Swim Bladder- 
Lungs and Air Ducts — The Lungs — Summary of the Respiratory 
System — Fishes — The Air-bladder and Accessory Organs of Respira- 
tion — Dogfish — Amphibia — Reptilia — Birds — Mammals — 
Accessory Respiratory Apparatus 333-347 


THE CIRCULATORY SYSTEM— Detailed Studies— The Heart— The 
Vascular System — Development — The Arteries — Aorta and Aortic 
Arches — Arteries of the Dorsal Aorta — Visceral Arteries — Somatic 
Arteries — The Veins — Summary of the Circulatory System — Amphioxus 

— The Lymphatic System — Fishes — Amphibia — Reptilia — Birds — 
Mammalia 348-381 


THE UROGENITAL SYSTEM— The Pronephros— The Mesonephros— 
The Mesonephric Duct — The Metanephros — The Urinary Bladder — 
The Reproductive Organs — The Reproductive Ducts — Oviducts — 
Organs of Copulation — Adrenal Organs — Summary of the Urogenital 
System — Fishes — Dogfish — Amphibia — Reptilia and Aves — Mammalia. .382-402 


THE MUSCULAR SYSTEM— The Visceral Muscles 403-410 


THE NERVOUS SYSTEM— The Spinal Cord— Flexures— Neuromeres— 
Meninges — The Brain — The Cerebrum — Brain Table — Telencephalon — 
Diencephalon — Epiphysial Structures — Mesencephalon — Rhombence- 
phalon — The Cerebellum — Medulla Oblongata — Telae Chorioideae — 
Summary of the Brain — Amphioxus — Cyclostomata — Pisces — Dogfish — 
Teleosts — Amphibia — Reptilia — Aves — Mammalia — The Organs of 
Special Sense — The Ear — The Nose — The Eye — The Peripheral 
Nervous System — The Sympathetic Nervous System — The Cranial 
Nerves 411-466 





BEFORE beginning the work in Introductory Embryology it is 
quite essential that the student turn back to earlier chapters and 
re-read what is said there on mitosis, fertilization, and the histology 
of the frog. Such a review will lay a foundation for the detailed study 
of the following pag-es. 

When Comparative Anatomy is taken up in the next semester's 
work, it will be found that the Haeckelian law of biogenesis (also called 
the "recapitulation theory"), although untrue in its usual application, 
is a very convenient supposition in that it makes many points clear if 
we accept it as a working hypothesis. This so-called law is defined as 
follows : "All anim^als, during their embryonic period, pass through 
the same adult-stages that the various members of the race to which 
they belong, have passed." For practical purposes it is necessary to 
keep this theory in mind in the study of Embryology ; for, it is the 
simplest way of bringing home to the student the fact, that in any 
biological study that is to be scientific, one must first study the more 
simple organisms and then compare such simple forms with those that 
are more complex — the so-called higher forms. 

All living animals pass through a quite similar stage of development 
in their embryonic period, so that the next succeeding higher form 
practically possesses everything that the immediately next succeeding 
lower form possesses, plus something additional. And it is this "plus 
something" that we are trying to arrange in proper order when we 
study embryology. 

The value of this is not always clear to the student. However, if 
he will remember that a human being and a chick pass through quite 
similar stages during their embryonic periods, the human being, how- 
ever, developing further, he can understand how an obstruction may 
prevent any individual part of an organism from receiving the proper 
nourishment and environment, and thus cause such part to cease devel- 
oping, and thereby to produce what is called a rudimentary structure. 
(Fig. 250.) 

While all animals differ slightly from each other, there are certain 
type-forms in which the greatest differences can be clearly observed. 
Such type-forms, as commonly used in the laboratory, are the dogfish, 
as a representative of the cartilaginous fishes; the frog, as an example 
of amphibia ; the chick, or pigeon, as an example of birds ; the turtle, 


Embryology of the Chick 

as an example of reptilia ; and the cat, rabbit, or pig, as an example of 
the mammals. 

As we have been using the frog as a norm, or standard type, with 
which 'to compare the other forms studied, it would probably seem best 

Fig. 250. 
There is a membrane covering the pupil of the eye which, in man, normally dis- 
appears when the embryo is seven months old. In the case here shown portions of 
the membrane have persisted as an irregular network over the pupil. Such 
persistent structures are called rudimentary. (From a drawing lent by Dr. G. N. 

to begin Embryology with that animal. However, for the same reason 
that the frog was used as an introductory subject for study (because 
it can be procured easily and because it is a fairly complex form which 
possesses structures with which the student is already familiar), so, the 
hen's egg, which is much larger than that of the frog, can also be 
obtained easily and is already somewhat familiar to the student. In 
addition to this, the chick embryo develops upon the surface of the yolk, 
which makes the various germ layers xery distinct, and serves much 
better than the frog as a beginning-type..j 

The first and foremost point in the study of Embryology is accuracy 
of observation; the second is the obtaining of a clear concept of what 
has been observed; and the third is to show by drawings that the first 
and second points have been fully assimilated. 

There is considerable need for legitimate imagination in embryo- 
logical work, because the entire study of Embryology is for the purpose 
of giving the student a more or less comprehensive idea of the process 
through which, and by which, all the organ-systems in the body of 
living things have come to be what they are. The study of Embryology 
is, therefore, different from later work in pure anatomy, where each 
structure is definite, and where such structure is studied only after it is 
completely formed. In Embryology we see the beginnings and develop- 
ment of these later anatomical structures. 

One should first take the complete embryo, and get a good grasp of 
the general structure. Then, sections must be cut at various intervals 
and studied microscopically. It must never be forgotten, however, that 
our imagination must constantly remind us that there are three dimen- 
sions to the living animal, and that what we are looking at in a section. 

Development of the Embryo 13 

is but a series of still pictures, and that there is little value or meaning 
in such observation unless one can, with imagination and logic, plus 
preceding biological knowledge, build up a completed structure, so that 
the mind's eye can see the entire animal as it actually exists. 

It must be remembered at this point that events which have taken 
place in the past, are the cause, or causes, of events that are now taking 
place, and that will take place later. This is as true in Embryology as 
it is in such a field as history, for example. This means that the various 
events of development are caused by preceding developmental events, 
and that these cause later steps in development in turn. 

Another important point for the student to remember is that he 
must not only be able to recognize histologically the type of cells he 
may find in the section he is studying, but he must know the definite 
location in the complete embryo from which his section is cut. 

The complete bird-like form of the chick can be clearly seen before 
the eigth day of incubation because all the principal changes have taken 
place by that time. It will, therefore, be understood that these changes 
are rather minute in their origins, for the eight-day embryo is only about 
seven millimeters in length. During, and after the eighth day, the 
changes which taice place are primarily enlargements, or growth, of 
portions already present. 

In the study of Embryology we are not only interested in the devel- 
opment of the chick from the ^gg, but we also wish to know how the 
egg came into existence. 

The hen's egg is usually said to be a single cell. This is, however, 
only true if the egg is unfertilized. 

As birds' eggs are laid with shells upon them, it is necessary that 
fertilization take place before the shell is formed. Fertilization in these 
cases is internal. It takes about 22 hours for the egg to have the layers 
of white laid down, and for the shell to surround it. (The layers of 
yolk are laid down before ovulation.) If the egg has been fertilized, 
the warmth of the mother's body has already caused development 
throughout these hours, so that by the time the egg is laid, the little 
chick is already approximately one, or one and a half, days old. There 
is a variation in the age because, if the hen's egg is ready for laying 
during the main part of the day, it is laid then, but if it is not ready 
for laying until, let us say, about four or five o'clock in the afternoon/ 
it is retained within the mother's body until the following day, thus 
causing some embryos to be developed from ten to fifteen hours more 
than others. 

The so-called spoiling of eggs is usually due to the fact that the 
embryo chick dies and then decays. 

In birds, where the eggs leave the mother's body, the yolk must 
be quite large in order to furnish sufficient food for the embryo during 
the two or three weeks intervening between the time the egg leaves the 
mother's body and the time of hatching. In mammalian forms, the 

14 Embryology of the Chick 

egg remains extremely small because the nourishment of the embryo is 
derived directly from the mother. 

During- the very first day of incubation the outlines of the embryo 
are defined. During the second day a rather complicated series of folds 
appear, separating- the embryo from the yolk. The embryo, however, 
remains in contact with the yolk-mass by a narrow stalk. The circula- 
tory system now develops, through which nourishment is carried from 
the yolk-mass to the embryo. Embryonic membranes and appendages 
appear during the second and third days of incubation. These assist in 
respiration and also in forming a larger area from which the food supply 
may be brought from the yolk to the embryo. 

Development usually begins at the head end and extends tailward, 
so that the brain and other head structures are often quite well devel- 
oped when there is little semblance of any other well-defined structure 
toward the tail end. The chick usually makes a small opening in the 
egg shell on about the twentieth day of incubation, and from then on 
the lungs actually take in air and begin their regular external work, 
while on the twenty-first day the chick breaks entirely through the 

With this introductory general outline, we shall take up the study 
of the egg itself, working backward to its very simplest cell origin in 
the mother's ovary. 


The true ovum (Fig. 251 A, v.), or egg-cell proper, is the large 
yolk or vitellum. This is surrounded by a tough vitelline membrane. 
The end of the ovum, where the embryo is to develop, is called the ani- 
mal pole. It is nearly free from yolk, and appears at the time of laying as 
a circular whitish area, known as the blastoderm (Fig. 251, b), and 
measures from three to four millimeters in diameter. As the animal 
pole is not so dense as the surrounding material, it is always found on 
top of the yolk, no matter which way the entire egg is turned, provided, 
of course, that the yolk is free to rotate. 

The more central portion of the animal pole is rather translucent, 
or pellucid, and, therefore, is called the area pellucida (Fig. 251 A, c). 
This central portion is surrounded by a whitish, or opaque, region called 
the area opaca. The yolk itself is called deutoplasm, and is divided into 
two types of material, white and yellow yolk. The white yolk is thickest 
in the region just below the blastoderm, where it is shaped like a flask, 
as shown in the figure. It extends to the center of the yolk. It will be 
noticed that the yolk is thus arranged in various concentric layers. A 
layer of thick yellow yolk alternates with a thinner stratum of white 
yolk. The two types of yolk differ in physical characteristics and in 
chemical composition. 

The vitellus, or true egg-cell, alone is formed in the ovary. Such 
structures as develop within the ovary proper are called primary. 
Structures, such as chorionic membranes (found in most of the higher 

Development of the Embryo 


- — Area opaca 
Area pellucida 

Fig. 251. 

Semidiagrammatic illustration of the hen's egg at the time of laying. A. 
Entire "egg." £. ^Diagram of a vertical section through the vitellus or ovum 
proper, showing tlie concentric layers of white and yellow yolk, a, Air chamljer; 
ac, chalaziferous layer of albumen; ad, dense layer of albumen; af, fluid layer of 
albumen; b, blastoderm; c. chalazae; /, latebra; nl, neck of latebra; P, nucleus of 
Pander; pv, perivitelline space; smi, inner -layer of shell membrane; smo, outer 
laj'cr of shell membrane; v, vitellus or "yolk"; vm, vitelline membrane; wy, layers 
of white yolk; yy, layers of yellow yolk. C. Surface view of Blastoderm of un- 
incubated hen's egg. (A and B, after Marshall; C, after Hertwig.) 

forms), are known as secondary structures, while those particular 
regions which are formed by accessory reproductive organs, such as the 
white of the egg and the shell, are said to be tertiary structures. The 
white of the ^gg is composed of albuminous matter which is chemically 
quite complex. It will be remembered that the protoplasm in all living 
cells is largely albuminous. 

Toward each end of the newly-laid tgg, one finds a dense, opaque 
twisted cord, extending through the white of the ^gg from opposite sides 
of the yolk toward the apices of the shell. These twisted cords are 
called chalazae (Fig. 251A, c). They are continuous with a very 
thin, dense layer of albumen surrounding the yolk. This thin layer is 
called the chalaziferous layer. It is generally assumed that the chalazae 
assist in holding the yolk in position, though this has been disputed by 
several biologists, primarily because the ends nearest the shell are not 
attached. Immediately outside the chalaziferous layer there is another 
thick, dense layer of albumen, and superficial to this is a still thicker 
layer of a more fluid albumen. The hard-boiled ^gg in which the albu- 
men has coagulated lends itself well for the observation of these various 
layers. Usually, in observing such hard-boiled eggs the albumen is seen 
to be arranged in spiral sheets. 

The ovoid shell which surrounds the entire tgg is quite resistant to 
gradually applied pressure, but easily broken if the blow be sharp. The 
shell in turn is covered superficially by a thin cuticle perforated by many 
pores. The main substance of the shell is made up of loosely arranged 


Embryology of the Chick 

particles of carbonates and phosphates of calcium and magnesium. The 
inner surface of the shell is composed of a thin but dense layer of 
inorganic salts. After the shell has dried, it is quite porous, thus making 
the passing of gases and water-vapor quite easy. 

There is a tough shell membrane lining the inner portion of the 
shell. It is composed of a double sheet of fibrous connective tissue 
which separates at the blunt end of the ^gg into an air space, becoming 
larger as time goes by. 


One obtains a more thorough 
the tgg in the ovary if a review 

Fig. 252. 
The reproductive system of the fowl. The 
figure shows two eggs in the oviduct, whereas 
normally only one egg is in the oviduct at a 
time, b, Blastoderm; c, cicatrix; cl, cloaca; 
da, dense layer of albumen; /, empty egg 
follicle from which the ovum has escaped; g, 
glandular portion of oviduct; i, isthmus; m, 
mesovarium; o^^-o^, ovarian ova in various 
stages of growth; O^, ovum in upper end of 
oviduct; Oj, ovum in middle portion of oviduct 
(the oviduct has been cut open to show the 
structure of this ovum) ; ps, ostium or infundi- 
bnlum; ov, ovary containing ova in various 
stages of growth; r, rectum; u, uterus; v, 
vitellus; w, ventral body wall, opened and 
reflected. (From Duval.) 

understanding of the development of 
of the entire reproductive organs is 
undertaken. The reproductive or- 
gans of the fowl do not develop 
equally on each side, though they 
begin developing symmetrically. 
The right ovary ultimately degen- 
erates, and so far as we know does 
not function. The left ovary (Fig. 
252) and oviduct alone carry on the 
work of the organs. The left or- 
gans, therefore, become quite large. 
A microscopical section of the 
ovary shows this organ to be com- 
posed of a great quantity of ova, 
each ovum being contained in a 
Graafian follicle (Fig. 253). The 
ovary itself is suspended from the 
dorsal abdominal wall by a double 
fold of the peritoneum called a 

In the hen, the ova vary in size 
from a very small cell up to the full 
sized yolk. The oviduct is large, 
thick-walled, and muscular, being 
convoluted, and having a different 
structural form in the different 
parts. The oviduct carries eggs 
from the ovary to the uterus. The 
abdominal opening of the oviduct is 
rather wide, flaring, and funnel- 
shaped, and comes in close contact 
with the ovary. This opening is 
called the ostium or infundibulum, 
or the fimbriated opening. This last 
name is due to its fringe-like mar- 

Development of the Embryo 


gin. This region of the oviduct is thin and muscular and lined with 
cilia. The oviduct proper, into which the ostium leads, is known as the 
convoluted glandular portion, which is followed by a short, third portion 
called the isthmus. It is after passing through the isthmus that the Qgg 
enters the so-called uterus, which is merely a dilated portion of the 
glandular tube. The uterus in turn opens into a short terminal region, 
a rather thin-walled vagina, and this again opens into the cloaca, just 
dorsal to the opening of the rectum. 

It is easier for the student to understand a developmental history 
of the egg if it be thought of as passing through three periods. First, 
from the beginning of the development of the ovum to the time of ovula- 
tion. Second, from the time of ovulation through the period of fertili- 
zation, and third, from the beginning of cleavage to the time the egg is 

First Period. (From the beginning of the development of the egg 
to the time of ovulation.) Most animals produce a large number of eggs 
within a very brief period, while in the hen there is a long period of egg 
formation and laying, which extends over several months, after which 
there is a period of almost complete cessation. Undoubtedly the reason 
for this is that, when an egg becomes so large as that of the hen, which 

You^g follicle u'ith^'um. rcquircs so much food in its 
making, it is a considerable 
drain upon the animal, and 
secondarily, there isn't room 
enough in the body of an 
animal no larger than a hen 
for many eggs of such size. 
However, the fact that the 
hen's ova develop in the way 
they do, makes it possible 
for us to observe almost a 
complete succession of de- 
velopmental changes from 
the minute forms up to the 
fully developed egg. 

In our course of general 
Biology, we learned that very early in an organism, especially in triplo- 
blastic forms, the germplasm and somatoplasm differentiate. A few 
cells are set aside in the innermost portion of the body of the growing 
embryo for reproductive purposes. The development of the germplasm 
in the growing embryo is called oogenesis in the female and spermato- 
genesis in the male. 


The process by which the eggs, already present in the ovary of the 
new-born chick, originally came to be what they are, is known as 

Fig. 253. 

Section from ovary of adult dog. The more or less 
star-shaped figure on the right is a collapsed follicle with 
its contents. Below and at the right are seen the tubules 
of the Parovarium. (After Waldeyer.) 


Embryology of the Chick 

oogenesis. The first event in oogenesis is known as the multiplicaticn 
of the oogonia. This occurs during- the embryonic period of the animal. 
There are two types of cells which develop from the original primary 
cells set aside for reproductive purposes. How and why these differ- 
entiate in the way they do, we do not know, but we do know that there 
is a differentiation. 

As soon as these original cells begin to divide, some of them develop 
into centrally located eggs or ova (Fig. 254), while others, known as 
germinal epithelium, surround the more centrally located ova and form 
a sort of case, or capsule, around them. The primitive egg surrounded 
by this epithelial case is known as an oogonium. Some of the primitive 
eggs leave the epithelium and pass into the stroma of the ovary. There 
they degenerate. Those remaining, however, begin enlarging even while 
they are dividing and multiplying. The epithelial cells also divide very 
rapidly, forming long strands or cords which in turn extend into the 
stroma. There comes a 
time when these primitive 
ova, or oogonia, stop multi- 
plying; they are then called 
primary oocytes. At this 
time the strands, or cords, of 
germinal epithelium break 
up into little groups, some- 
times called nests. Each 
nest consists of a single pri- 
mary oocyte (Fig. 255) sur- 
rounded by a number of the 
original epithelial cells. 
These latter cells form a 
definite case surrounding the 
oocyte. The case thus 
formed is called the primi- 
tive egg follicle. This final 
arrangement takes place 
within a few days after 
hatching. It will thus be 
seen that all the eggs which 
enlarge, ripen, and pass out of the ovary are merely enlarged and devel- 
oped primary oocytes. 

Both the nucleus and the cytoplasm of the egg cell now begin to 
enlarge, and yolk granules are laid down all about the centrally located 
nucleus as well as throughout the cytoplasm, except in the peripheral 
region. This region remains comparatively free from yolk. At the point 
where the ovum, or follicle, is attached (Fig. 256), there is a thicker 

Section of the Germinal Epithelium and Adjacent Stroma 
in a Chick-Embryo. 
g.ep., germinal epithelium forming a thickened ridge- 
like projection; pr-ov., primitive ova of various sizes, some 
in the germinal epithelium and others somewhat beyond 
the limit of this epithelium; St., strands of cells which have 
grown from the germinal epithelium, and one of which 
appears connected with an enlarged primitive ovum. (From 

Development of the Embryo 


portion in the periphery known as the germinal disc or spot. As soon 
as the ovum reaches a diameter of about five-tenths of a millimeter, the 
nucleus migrates into the germinal disc, v^here it remains as long as the 
egg continues in the ovary. An important point to remember is that 
the animal pole of the ovum is toward the attached surface, that is, at 
the point where the nucleus is located. 

From this time onward, the yolk accumulates very rapidly. The 
surface of the ovum is in the form of a zona radiata (Fig. 256, B), in 
which there are many pores through which nutritive substances may 
easily diffuse from the follicle cells. These follicle cells may, therefore, be 
called nurse cells. 

When the follicle has completed its growth, it becomes somewhat 
membranous. Directly opposite its point of attachment there are very 

Follicular cavily. 

Fig. 255. 

Young Mammalian Oocyte surrounded 
by a single layer of Follicular Cells. 
(Van der Stricht.) 

Showing attraction-sphere, centrosome, 
and mitochondria. 

Corona radiata. 
Zona pellucida. 
Germinal spot. 

Germinal vesicle or 


Fig. 256. 
d, ripe Mammalian Graafian follicle. B, ovum. 

few blood vessels, and it is at this point that a modification takes place 
in the appearance of a band, known as the cicatrix. It is at the cicatrix 
that the follicle ruptures to permit the escape of the tgg into the 

The nucleus lies flat against the vitelline membrane, and becomes 
very large just before the tgg leaves the ovary. It is then called a 
germinal vesicle, because the chromatin condenses, which leaves the 
nucleus appearing as a large clear hollow spot. The nuclear wall now 
breaks down and forms the first polar spindle. This rotates into position 
and the primary oocyte is ready for its first maturation division, and 
later, for ovulation. 

20 Embryology of the Chick 

Second Period. (Ovulation, maturation, and fertilization.) 

The coordination of different functions in the body is well shown 
by the fact that at about the time a completed egg is ready to pass into 
the oviduct, the region of the ostium of the oviduct becomes very active 
and actually seems to grasp the ovarian follicle which contains the pri- 
mary oocyte. This may be due to muscular, or ciliary, action or it may 
be a combination of both. The follicle then ruptures, permitting the 
egg to be thrown out. It seems that the pressure exerted by the con- 
traction of the fringed end of the ostium may have something to do 
with such rupture. The throwing out of the eggs from the follicle is 
called ovulation. The oocyte always enters the infundibulum of the 
oviduct with its chief axis transverse to the long axis of the oviduct, and 
throughout its entire passage down the tube, this relation is retained. 

After the sperm have been injected into the female, they make their 
way up the oviduct toward the ovar}^, seeming to gather at its end. 
They may remain alive and function for at least two weeks, sometimes 
even longer. It will thus be noted that as soon as the egg has been 
discharged from the follicle and has been taken into the oviduct, there 
are millions of sperm floating about in the fluid surrounding it. A 
single egg of the hen, unlike that in most animals, has from five to 
twenty-four spermatozoa enter it. Such a process is known as 
polyspermy. Polyspermy is abnormal in most animals, but it is the 
normal condition in the hen. The egg is now fertilized. The sperm 
apparently affords the stimulus which causes the egg to begin dividing 
and to form an embryo. 

The egg, after the entrance of the various spermatozoa, is not yet 
completely mature. A process of maturation now takes place.* This 
means that the egg divides into a larger and a smaller portion, both of 
which portions may again divide into two parts. All of the smaller 
portions degenerate, one large portion alone developing into a complete, 
fertilized, hen's egg. The purpose of the small polar bodies (as the 
degenerating portions are called) is to throw off one-half of the 
chromatin in order that the new-born young may be a normal indi- 
vidual like its parent, as explained in our studies of mitosis, maturation, 
and genetics. 

After the second maturation division, the remaining nucleus unites 
with a single sperm nucleus to form the first cleavage spindle, and the 
egg is now ready to begin dividing and form a true embryo. 

Third Period. (From the beginning of cleavage to the time the 
egg is laid.) 

It must be remembered here that the fertilized egg, which is to 
become the embryo, is present in the hen's body quite a number of hours 
before the egg is laid ; in fact, from one to one and a half days before the 
various layers of white and shell have encircled it. The heat from the 

*In many animal-forms maturation takes place before ovulation; in some it begins before ovula- 
tion but is not completed until some time after. 

Development of the Embryo 


mother's body has caused the embryo to begin to form, so that by 
the time the egg- is laid, the embryo is already many hours old. It is, 
therefore, essential that the student understands in detail, exactly what 
has already happened in the mother's body before the egg passes to 
the outer world. 

The first cleavage furrow can be seen about three hours after the 
ovum has been discharged from the follicle. During this period the egg 
has passed along the entire glandular portion of the oviduct. The glands 
themselves have secreted the most dense portions of albumen and also 
the chalazae. The yolk was already laid down before ovulation. The 
egg is carried along principally by peristaltic action of the walls of the 
oviduct. Then, as the egg itself rotates, the germ disc comes to describe 
a spiral path, which explains the spiral arrangement of the albumen 
around the yolk. The egg then traverses the isthmus for approximately 
an hour, where the shell membrane is secreted over the dense albumen. 
The fluid layer of albumen is secreted both in the isthmus and the 
upper part of the uterus. The fluid layer of the albumen passes through 
the shell membrane which has already been laid down, and it takes 
from five to seven hours after the egg enters the uterus before this is 
completed. But, before this takes place, the shell substance itself has 
already begun to be laid down on the shell membrane. Usually twelve 
to sixteen hours are necessary to complete the passage through the 
uterus and vagina. At the end of this time twenty-one to twenty-seven 
hours have already elapsed since ovulation took place. Gastrulation has 
begun, and the egg is laid. 

We have already mentioned that, if the egg reaches the vagina, 
ready to be laid, during the main portion of the day, it will be laid on 
that day. If, however, it should be ready for laying after four or five 
o'clock in the afternoon, it will be retained in the vagina until the fol- 
lowing day, thus causing some embryos in freshly-laid eggs to be 
approximately twelve hours older than others. It is for this reason that 
there is always considerable variation, even when eggs have been incu- 
bated for the same number of days. 



Hours after 


to 3 

Location in 



Action of Oviduct 

Reception of Ovum 

Secretion of chala- 
zae, chalaziferous 
and dense albumen 

Action of Germ Disc 

Maturation and Fer- 

First cleavage fur- 


Embryology of the Chick 

3 to 4 


Secretion of shell 

Formation of eight 

membrane and 


fluid albumen. 

4 to 21 (27) 

Uterus and 

Secretion of shell 

Gastrulation begun, 


and fluid albumen. 

or c o m p 1 e ted if 

Retention prior to 

egg is long re- 



With what has just been said in mind, the developmental processes 
of an embryo become more understandable. The unicellular germ disc 
is composed of a very definite area at the animal pole. The disc itself 
is about three millimeters in diameter, and less than five-tenths milli- 
meters in thickness. Directly beneath this disc, there is a merging of 
the protoplasm with the white yolk. This well-marked region is called 
the nucleus of Pander (Fig. 251, P), and this connects the central white 
yolk by a narrow stalk called the latebra. It is necessary to study all 
the figures carefully to understand these and successive terms, and to 
grasp the relationship of each to the other. 

There are two regions in the disc itself : the larger central portion 

Fig. 257. 

Cleavage. Upper Row, Amphioxus. (After Hatschek.) 1, Unfertilized egg; 2, 
stage of two blastomeres ; 3, stage of four blastomeres; 4, stage of eight blast- 
omeres; 5, stage of seventy-two blastomeres; 6, section of blastula; p.b., polar body. 
Middle Row, Frog. B, segmentation cavity, v, nucleus. Lower Row, Hen's tgg. 
(After Patterson.) Surface views of the blastoderm and the inner part of the 
marginal periblast only. The anterior margin of the blastodisc is toward the top 
of the page. A. Two-cell stage. About three hours after fertilization. B. Four 
cells. About three and one-fourth hours after fertilization. C. Eight cells. About 
four hours after fertilization. D. Thirty-four cells. About four and three-fourths 
hours after fertilization. E. One hundred and fifty-four cells upon the surface; 
the blastoderm averages about three cells in th'ickness at this stage. About seven 
hours after fertilization, ac, Accessory cleavage furrows; m, radial furrow; p, 
inner part of marginal periblast; iac, small cell formed by the accessory cleavage 

Development of the Embryo 


which is to form the blastoderm proper, and the narrow denser area 
known as the periblast, which forms the outer margin. The periblast 
is continuous with the membrane covering the yolk, peripherally. 

In the center of the germ disc, the first cleavage furrow appears. 
(Fig. 257.) It is short and shallow, running about one-half the diameter 
of the disc. We do not know whether the first cleavage extends 
directly through the central portion of the embryo. The main embry- 
onic axis lies almost at right angles to the long axis of the whole egg, 
the head end of the embryo being directed toward the left when the 
sharp end of the egg is held pointing away from the observer. The first 
cleavage plane does not seem to have any definite relation to either of 
these axes. 

The second cleavage is also vertical and almost at right angles to 
the first, so that we have four adequal cells, all, however, incomplete. 
The third cleavage appears about an hour after the first. This is usually 
parallel with the first. It divides the disc into two rows of four cells 
each. This cleavage may be quite irregular in form, and from now on it 
is impossible to tell exactly how and when, in relation to time especially, 
these egg cells divide. Consequently, after they have divided and formed 
sixteen cells, all of these cells are very irregular, and there is a tendency 
in the fourth cleavage plane to separate the eight cells into a central and 
a marginal group. 

Fig. 258. 

Diagrams showing the blastulae: A, of Amphioxus; B 
of frog, and C, of chick; D, blastodermic vesicle of mammal. 
(After Semon.) 

The group of central cells becomes circumscribed and must not 
be confused with the marginal cells which remain incomplete both 
below and distally, retaining their connection with the periblast. These 
central cells have been separated by a horizontal cleavage plane, and 
this cleavage plane separates the more superficial cellular elements from 

24 Embryology of the Chick 

the underlying undivided substances, leaving a little space, which is the 
beginning of the segmentation cavity or blastocoele (Figs. 257, II B, and 
258). The undivided substance beneath is called the central periblast, 
the original periblastic region being now known as the marginal peri- 
blast. Both of the periblastic regions retain their connection with each 
other peripherally in the deeper regions of the marginal cells. 

The question that may arise here is, "What has become of the 
accessory or supernumerary spermatozoa?" Between the time of fer- 
tilization and the first cleavage, these have formed nuclei which migrated 
to the outlying portion of the blastodisc. There they probably divided 
once or twice to form small groups of daughter nuclei. There even 
seems to be an attempt of the cytoplasm to divide, and sometimes short 
superficial growths are actually formed. These are called accessory 
cleavages. They can be seen during the four and eight cell stage, 
usually radial in direction, lying just across, or outside, the margin of 
the blastodisc. No true cells, however, are formed by such cleavages. 
The accessory sperm nuclei all degenerate rather rapidly, the accessory 
cleavages fading away, so that by the time the embryo has reached the 
thirty-two cell stage, no traces of these accessory structures can be 
found at all. 

As cleavage continues, the number of central cells increases very 
rapidly by the marginal cells, dividing and being added to the central 
cells, although the central cells divide likewise. This latter multiplica- 
tion is very rapid, the cells diminishing in size. For example, cleavages 
appear in the central cells, causing the roof of the blastocoele to become 
several cells in thickness. No cells are added to the germ disc from 
the floor of the segmentation cavity. The continual cutting off of 
central cells from the marginal cells causes these latter to be consid- 
erably shortened, until finally they are limited to the extreme margin of 
the blastodisc only. 

After division has taken place so that two or three hundred cells 
have been formed, there are intercellular furrows extending out into the 
marginal periblast. Up to this time, there have been no nuclei what- 
ever in either central or marginal periblast, but two areas, which are 
continuous, now become converted into a nucleated syncytium. Our 
knowledge of this developing process comes from the study of the 
pigeon. It has not been worked out in the chick. The process is some- 
what like this : The marginal cells have become spherical in form, by 
having the central cells cut off from them. Their nuclei now divide, 
although the cytoplasmic divisions are either completely lacking or do 
not completely divide. The free nuclei, therefore, become quite exten- 
sive in the margin of the blastodisc, and as these nuclei continue multi- 
plying, they wander off into the marginal periblast so that nuclei are 
scattered about quite thickly, though the structure itself is non-cellular. 
Some of the nuclei also migrate inward below the blastodisc, so that 
the central periblast is likewise converted into a nucleated structure 

Development of the Embryo 35 

with the exception of the middle area above the nucleus of Pander. 
This area continues to remain free from nuclei ; in fact, what is later 
to be known as the germ wall, is partly composed of the nucleated rim 
of the periblast. 

The blastoderm, which is rather circular, extends radially, both on 
account of the growth of its own cells, and by the addition of cells from 
the marginal periblast. The original region of the blastodisc becomes 
thinner and transparent. It is then called the area pellucida. The cir- 
cular margin, which is derived from the periblast, is called the area 
opaca. The ring-like periblast keeps on growing, while additional nuclei 
are formed peripherally. At the same time, the periblast is contributing 
cells to the blastoderm also, so that the blastoderm steadily increases 
in diameter. The inner nucleated margin of the periblast, which later 
becomes cellular, contributes to the later extra-embryonic tissues and is 
called the germ wall. The cells of the blastoderm later extend periph- 
erally so that they overlap the inner margin of the germ wall, to form 
a narrow region, transitional between pellucid and opaque areas. 

It should be noted here that the lower surface of the periblast is 
directly continuous with the yolk mass, and peripherally it is continuous 
with a very thin superficial tissue of protoplasm. This latter is also 
often referred to as a part of the germ wall. 

As soon as the blastoderm has become thinned out as mentioned 
above, the blastula stage is completed. 

It is well at this point partially to summarize the development 
through the morula and blastula stage before taking up gastrulation. 


While text-books usually speak of an "end" to the segmentation 
process, it must not be supposed that the cells of the embryo stop 
dividing. The whole process is continuous, and the word "end" here 
means only that the general process of cell-division is now "general" 
no longer, but that differentiation begins. The ending of the segmenta- 
tion stage means only that one can from this period on, find a grouping 
or aggregation of cells which are not all alike. 

In eggs in which there is but little yolk (therefore not in birds' 
eggs), the segmentation results in a rounded, closely packed mass of 
embryonic cells (blastomeres), called a morula. This name has been 
given such a cell mass because it resembles a mulberry. This morula 
stage, in eggs with little yolk, corresponds to the stage at the "end" 
of segmentation in the chick embryo. At this time the embryo is a 
simple disc-shaped mass of cells, several layers in thickness. This whole 
mass is the blastoderm. It lies closely applied to the yolk. 

The cells in the center of the blastoderm are smaller and quite 
clearly defined, while the surrounding or peripheral cells are flattened, 
larger, and in more intimate contact with the yolk beneath. 


Embryology of the Chick 


The "chick embryo remains in the morula stage for a very short 
period, then there is a rearrangement of cells preliminary to the blastula 
formation. First, a cavity forms beneath the blastoderm due to the 
smaller central cells separating from the underlying yolk. The outlying 
cells remain attached. This space is called the segmentation cavity, or 
blastocoel, while the marginal area of the blastoderm, which remains 
attached to the yolk, is called the zone of junction. As soon as the 
segmentation cavity is thus formed, the embryo is said to be in the 
blastula stage. 

From Figure 259, which shows only the blastoderm and a portion 
of the yolk (the yolk being about three feet in diameter at this mag- 
nification), a good understanding may be had of the difference which 
a larger amount of yolk makes in the blastula-formation. 

In eggs with little yolk, a definite morula or solid sphere of cells can 
easily form, which may then develop into a hollow sphere or blastula. 
But in eggs with a large quantity of yolk, as in the pigeon and the chick 
(Fig. 258), the blastomeres are forced to grow on the surface of the 

area opaca 


Fig. 259. 

Diagrams to show various stages in the gastrulation of a bird embryo. In 
the surface-view plans, the blastoderm is supposed to be transparent so the under- 
lying structures may be located. A, surface view of blastoderm, just before 
invagination; B, surface view of blastoderm, invagination well advanced; C, surface 
view of blastoderm at end of gastrulation; D, vertical section through blastoderm 
of stage represented in A; The plane of section is indicated by the line a-a in A. 
E, vertical section through blastoderm of stage represented in B. The plane of the 
section is indicated by the line b-b in B. F, vertical section through blastoderm 
of stage represented in C. The plane of the section is indicated by the lines c-c 
in C. (From Patten, after Patterson's figures for the pigeon.) 

Development of the Embryo 


yolk, which is the mechanical reason for the disc-shaped blastoderm 
being where it is arid what it is in the bird's egg. That is, if the large 
yolk of a bird's egg were removed and the blastoderm were allowed to 
assume the spherical shape which it would naturally take due to surface 
tension, there would be a decided similarity between the disc-shaped 
blastoderm and the ordinary morula stage of eggs with little yolk, such 
as in Amphioxus and in man. 

Not only does the great quantity of yolk make this change in the 
morula stage, but it is evident that a large amount of yolk does not 
permit a simple hollow sphere formation by any method of cell arrange- 
ment. Nevertheless, the central cells do separate somewhat from the 
yolk and form the slight segmentation cavity mentioned above. 

Imagining, now, that the yolk could be removed and the ends of the 
blastoderm drawn together, we should have a true blastula form of the 
simpler type. 


It is essential that one remember that a large quantity of yolk will 
make a considerable change in the process of gastrulation. The simpler 

Fig. 260. 

Gastrulation in egg with different quantities of yolk. 1-5, Amphioxus (little 
yolk); 6-8 Amphibian (moderate amount of yolk); 9-10, Birds (large amount of 
yolk); blc, blastocoele; future dorsal side; ect.. ectoderm; end., entoderm; ent. and 
ach., archenteron; bl[^., blastopore; y.p., yolk plug. (After various authors.) 


Embryology of the Chick 


t- ZJ. — J 

^pr ,,-•" 

1 — cw J 



J|3 4 


;^ /jtvit'-'y 

A to D. Diagrams illustrating the idea of 
confluence (concresence) as applied to the chick. 
The central area bounded by the broken line rep- 
resents the area pellucida; external to this is the 
area opaca, showing the germ wall (G. WS), zone, 
of junction {Z.J.), and margin of over-growth 
{M.O.), m.n.. Marginal notch. 

E to G, Diagrammatic relations of the germ 
layers at the time the primitive streak is formed 
by concrescence of the blastoporal margins. E, 
section of stage B; F, section of stage D; G, 
section through blastoderm of a 16 hour chick 
embryo. (^A to D from Lillie's "The Development 
of the Chick," by permission of Henry Holt & 

forms are brought about by an 
inpushing of the outer layer of 
the blastula as though one were 
indenting a rubber ball. This 
forms a two walled (ectoder- 
mal and entodermal) cup with 
a cavity in the center, called a 
gastrocoele. The opening itself 
is known as the blastopore 
(Fig. 260). 

In birds with a large 
amount of yolk, the blastula 
cannot indent completely into 
the blastocoele, due to the fact 
that the disc-shaped blasto- 
derm is not a true hollow 
sphere. The very small blas- 
tocoele formed between the 
blastoderm and the yolk, allows 
but little infolding. The blas- 
topore in the case of an in- 
dented sphere is relatively 
large. In the chick there is but 
a tiny blastocoele, while the 
blastopore is but a small cres- 
cent-shaped slit at the margin 
of the blastoderm (Fig. 251, C). 
This slit is to be thought of, 
however, as similar to the reg- 
ular round opening in simpler 
forms, which has been pushed 
together by the yolk not yield- 
ing. The infolding entoderm is 
also naturally compressed and 
flattened by the tiny blas- 
tocoele into which it can grow. 
In fact, the lower layer of the 
infolding entoderm seems to 
be prevented from growing 
normally by the unyielding 
yolk, and so is broken and lies 
on the yolk as scattered cells. 
These scattered cells then 
shortly disappear so that the 
yolk itself forms the floor of 
the gastrocoele. 

Development of the Embryo 29 

Figure 260 presents a diagrammatic scheme which makes it possible 
to see the general outlines of gastrulation in eggs with varying quan- 
tities of yolk. 

The zone of junction, where the peripheral region of the blastoderm 
remains attached to the yolk, is called the area opaca, because when the 
blastoderm is removed from the yolk-surface for laboratory study, the 
yolk is so closely attached to this region that it adheres to the blastoderm 
and renders the area more opaque. The more central portion, which 
has no yolk attached, is more translucent and is, therefore, called the 
area pellucida. 

The area opaca later differentiates into the following three more or 
less distinct zones (Fig. 261) : 

(1) The margin of overgrowth, a peripheral zone where rapid 
proliferation pushes the cells out over the yolk without their adhering 
to it. 

(2) The zone of junction, having an intermediate zone in which 
the deeper lying cells have no complete cell boundaries, so that they 
form a syncytium which blends (without a definite boundary) with the 
superficial layer of white yolk to which it adheres by many penetrating 
strands of cytoplasm. 

(3) The germ wall, an inner zone made up of cells derived from 
the inner border of the zone of junction, which have acquired definite 
boundaries and become more or less free from the yolk. Numerous 
small yolk granules are usually found in the germ wall, due to the fact 
that these were contained in the cytoplasm when they were still con- 
nected with the yolk as cells of the zone of junction. It is the inner 
margin of the germ wall which separates the area opaca from the area 

When the chick embryo is ready for gastrulation, there is a thinning 
of the blastoderm at the caudal margin with a consequent freeing of the 
blastoderm at the caudal margin from the yolk (Fig. 259, D). In a 
surface view, the crescent shaped gap in the posterior quadrant of the 
zone of junction marks the separation of the blastoderm from the yolk 
(Fig. 259, A). The blastopore is that region where the blastoderm is 
free from yolk and where it is likewise very thin. 

It will be remembered that cell proliferation is continuous through- 
out the entire blastoderm. The surface extent has now become much 
greater by a general spreading out of the peripheral margins over the 
yolk, but this extension, while taking place uniformly at the margins, 
varies at the blastopore. This being at the posterior free end of the 
blastoderm, the cells, as they proliferate, grow inward to form the ento- 
derm. Once this differentiation has taken place, the part of the margin 
forming this entodermal portion takes no further part in the peripheral 

30 Embryology of the Chick 

expansion, although this entodermal part grows back toward the center 
of the blastoderm, leaving the blastopore region behind. The marginal 
region continues to grow and soon encloses it, so that by the time the 
blastopore comes to close, it lies within the recomputed circle of the 
germ wall (Fig. 259, C). 



ALL that has been described so far has actually taken place before 
the tgg is laid. The real beginnings of a distinguishable embryonic 
area may be said to start with the primitive streak. While there are 
various theories as t6 just how this thickened streak is formed, the most 
logical and intelligible is that it is a thickening formed by the two lips of 
the blastopore meeting and growing downward. 

To make this clear, the student will remember that throughout this 
entire work, the blastula has been considered a hollow sphere com- 
posed of a single layer of cells, and the gastrula was this same hollow 
sphere after it had indented so as to form two layers. The opening 
where the indentation took place was called the blastopore. 

In the chick-embrvo we are to 

ff/uMi fsland 

think of this blastula, however, not 
as a sphere, but as sausage-shaped, 
Avith the indentation taking place 
from about the center of the long 
axis to one end. Thus we do not 
have a round blastopore, but an 
elongated one. And it is the closing 
of the lips along this elongated slit 
which forms the thickening called 
the primitive streak (Fig. 262). It 
is clearly seen at sixteen hours oi 
incubation, not only as a thickening, 
but as an indentation — the primitive 
groove — with ridge-like thickenings, 
Hanking each side and extending 
from the area opaca to almost the 
center of the blastoderm. The part lying closest to the area opaca is 
the caudal end, and the direction of the streak forms the long axis of 
the embryo. At the cephalic end of the primitive groove there is a 
deepening, called the primitive pit, and directly anterior to this the two 
lips of the primitive folds meet in the midline to form a small rounded 
elevation, known as Hensen's node. This node serves as the region of 
demarcation separating the fast disappearing primitive streak from the 
notocho;-d, which forms cephalad to it in the long axis of the embryonic 
area. The growth of the embryo is much greater headward than cau- 
dally or laterally, so that the anterio-posterior axis becomes considerably 

Fig. 262. 
Dorsal view of 16 to 20 hour chick embryo 
showing primitive streak, primitive groove, 
primitive node, beginning o£ neural groove, 
blood-islands, and extent of mesoderm. (After 


Embryology of the Chick 

The lips of the blastopore form a region of rapid cell proliferation, 
though all the cells look quite alike. Nevertheless, it is from these 
rapidly proliferating cells that the various germ layers are derived. 

Figure 263 shows an enlarged longitudinal, as well as a cross section 
of an early embryo. As. the lips of the blastopore grow closer and 
closer together, they finally fuse, forming the primitive streak. Ecto- 
derm and entoderm cannot be distinguished, but from the thickened 
approximation of the lips of the blastopore there is an inward growth of 


•*-* V9X.'.*W*>1 

Fig. 263. 

A. From medial longitudinal section through embryonic disk of Chick. Bonnet. 

B. From transverse section through Hensen's node- — germ disk of chick of 2 
to 6 hours' incubation. Duval. For lettering see Fig. C. 

C. From transverse section through primitive groove — germ disk of chick of 2 
to 6 hours' incubation. Duval, arc, Archenteron; ec, ectoderm; en., entoderm; 
l.b., lip of blastopore; p.g., primitive groove; y., yolk; y.p., yolk plug. 

a single layer of cells, now called entoderm, and from between these two 
layers some rather loosely arranged cells form a third layer, considered 
the primitive origin of what is later to be called mesoderm. 

At the same time this mesodermal outgrowth appears, the dipping 
down of the outer layer occurs to form the primitive groove. 

The three layers which have thus been established are very impor- 
tant because in all forms of animal life so far studied, there is a decided 
similarity in the origins and development of the various organ systems. 
Therefore, an understanding of the way the germ-layers and the organ 
systems arise, alone permits an understanding of the ever-increasing 
perplexities coming forth as these in turn develop further. 

In our study of comparative anatomy we shall see why it is that 

Origin- of the Mesoderm 


the nervous system as well as all outer coverings of the body are derived 
from the ectoderm ; why the lining of both digestive and respiratory 
organs comes from the entoderm ; and why the circulatory system as 
well as the blood, lymph, muscle, and connective tissue (except neu- 
roglia) are derived from the mesoderm. 

The primitive streak, relatively, seems to become pushed further 
and further tailward, but this is due to the greater growth in the cephalic 
region of the embryo. (Compare Figures 262 and 264.) 

The entoderm spreads out as a very definite layer of cells, and 
merges peripherally with the inner margin of the germ wall, even over- 
lapping it slightly. The little cavity between the yolk and this ento- 
dermal layer, which has been called the gastrocoele, will henceforth be 
known as the archenteron or primitive gut (Fig. 265). The student is 
not to look for a cavity in his sections, however, as the yolk in this 

region, by the very fact that it is separated 
from the entoderm and forms the floor of 
the primitive gut cavity, will not adhere to 
the embryo when it is removed for section- 
ing purposes. 

At eighteen hours of incubation the cell 
boundaries of the germ wall cannot usually 
be seen, though there are many nuclei and 
yolk granules, the latter in various stages 
of absorption. Because the nuclei^ of the 
germ wall arise by division of the nuclei of 
the cells lying at the margins of the expand- 
ing blastoderm, it is assumed they are 
instrumental in breaking up the yolk in 

Surface view of a twenty-one hour crl^m-nr-f^ r\f +Ua o4-*.,%r^1 ^-C ^t i- 

chick embryo, in which the head-fold advaucc ot the amval of the spreadmg 
and first pair of primitive mesoder- entoderm about the volk Sphere 

mal segments are present. (After <- i. xv, j' wixv oj^nv^ic. 

^"^^^•^ ^ _ At about twenty-two to twenty-three 

hours of incubation a pocket of entoderm can be seen in the anterior 
region by examining the whole mount, and focusing through the ecto- 
derm. This is the first formation of a gut floor in addition to the yolk. 
It is the yolk which has been answering that purpose up to this moment. 
This pocket forms the fore-gut. 

The mesoderm grows laterad and then extends cephalad, so that 
an area between the two cephalad growing portions of mesoderm is 
formed. This area is called the proamnion (Fig. 266, P) and is merely 
an open space, which must not be thought of as forming the later true 
amnion. It is to be noted primarily, because it permits a better study 
of just how the mesoderm grows in relation to it. It will be well to 
observe the difference in this space in eighteen and twenty-three hour 

As the mesoderm begins its growth where it does, there is none of 
it in the midline except posterior to the primitive streak; but immedi- 

Fig. 264. 


Embryology of the Chick 

ately on each side of the midline, the mesoderm is quite thickened, 
thinning out as it extends toward each side. The dorsal mesodermic 
plates are to develop from these thickened portions of the mesoderm, 
and as they will then segment, they are called segmental zones of 
mesoderm. The first somites will appear cephalad to Hensen's node, 
extending caudally along each side of the primitive streak and becoming 
less and less distinct. 

Aitiklcrm <ij tmin yi'Uit, dt^o 


'CampUlioii pi 

Fig. 265. 

From medial vertical sections through embryonic disk of lizard, 
showing five successive stages in gastrulation (Wenckeback. 

It is important to note here that the sheet-like layers of mesoderm, 
so characteristic in the mid-body region, do not extend to the head region 
of the embryo. The mesoderm of the head region develops from quite 
definitely organized layers immediately behind the future head. The 
reason that the mesoderm of the head is separate in origin from that of 
the remaining portion of the body, may be accounted for by the fact that 
the head is not segmented as is the mesoderm of the body-region. 


From the cephalic end of the primitive streak the rapidly prolif- 
erating cells extend in an anterior manner. In non-bird-like vertebrates. 

Origin of the Mesoderm 


the notochord extends from the region of the anterior lip of the blasto- 
pore, so it is assumed that this is also the case in birds. 

If the student will think of assumptions and incidents of this kind, 
and note the manner in which hundreds of such assumptions and inci- 
dents must be gathered from all angles and from hundreds of experi- 
ments by hundreds of different investigators, to make such a study as 
embryology possible, he will obtain at least some slight appreciation of 
what scientific investigation means and what scientific method means. 

In reading the literature on the subject, the student will note that 
probably most writers insist that the notochord develops from the ento- 
derm, though there are those who believe that it comes from either of 
the other two layers, and some even that it comes from all three. 







|| ' 


■ Z;' 

i "• 

J. A ... . 

■ "---:■■■■ 

S"-'v; ■ 


-'* s 

\\ -'.'u. 





Fig. 266. 


A. — Surface view of Embryo at the Twenty-third Hour of Incubation. 
A., anterior limit of head; AP., area pellucida; AV., area vasculosa B., 
border of mesoderm; C.A., yolk crescent; H., Hensen's node; P., proamnion; 
PP., primitive streak; PV ., mesoblastic somites; St., sinus terminalis bounding 
the vascular area; U., unsegmented mesoderm. /, region where the medullary 
folds have almost met to form the medullary canal. 

B. — Anterior part of the preceding figure more highly mag'nified to show 
details. A, ectoderm of anterior end of head; B, mesenchyme; C, subcephalic 
pocket: 1, region where the medullary folds will begin fusing to form medullary 
canal; 2, margin of the anterior intestinal portal; 3 and 4, posterior regions 
of medullary folds; 5, lateral limits of head region; 6, border of foregut. 
(From Duval.) 

In all forms studied, however, the notochord is not seen to arise 
from any definite layer, but it arises either at the same time the meso- 
derm does (Fig. 267), or from the undifferentiated growth of cells about 
the closed blastopore which gives rise to both entoderm and mesoderm. 

The notochord itself is a rod-shaped structure, circular in cross sec- 
tion, extending headward from Hensen's node. 


A thickening of the ectoderm at about eighteen hours' incubation 
causes a greater density along each side of the notochord. This denser 
area is several cells in thickness, and forms what is called the neural or 

36 Embryology of the Chick 

medullary plate. From Hensen's node caudad, the lateral portions of 
the medullary plates diverge into thickenings on each side of the primi- 
tive streak. 

At twenty-one to twenty-two hours the outer portions of the neural 
plate bend dorsally toward the midline and form the neural or medul- 
lary groove; the ridges thus formed are called the neural or medullary 
folds (Fig. 266, B). This is the first dififerentiation of the nervous 

After this period of incubation the denser portion which has formed 
by the cell differentiation mentioned above, is called the embryonal 
area, and the outer peripheral region of the blastoderm is called the 
extra-embryonic area, because from this extra-embryonic region arise 
those structures which are not part and parcel of the embryo itself, but 
serve as protective and nutritive layers. 

At this period the anlage of the head appears as a rounded elevation 
with a definite crescent-shaped head-fold, the first definite boundary of 
the growing embryo. 

It is Avell at this point to 

t WWmmMWLmM!MiM^ms>x .-^r^^m^ know what is to become of the 

mesoderm, so as to have several 

landmarks which will stand us in 
^'^^"^■'^^^^^B^^^^^S^^H^^^^ good stead. 

In the earthworm, it will be 
p.' 267 recalled, the entire animal is seg- 

Sagittal section through region of primitive mentcd, that is, COmpOSCd of 

node and caudal end of chorda! canal of guinea metamereS ; while in the frOg", 
pig (13>4 days after fertilization) to show be- ' ^ ^ ^S' 

ginning of notochordal cells and ectodermal cells in segflTientation S h O W S itSClf Ori- 
one layer. Ect., ectoderm; ent., entoderm; ch.c, . . . , - ^ 

chordal canal, dorsal and ventral wall closing manly lU the SplUal COlumU. 
lumen;, primitive pit, (After Ruber in The 
Anatomical Record, April 20, 1918.) Jj^ ^^^^^ Carthworm and frOg 

the segments are composed of an outer layer of ectoderm, an inner layer 
of entoderm, and a middle layer of mesoderm. 

When one speaks of metameres, one always means segments lying 
one behind the other, but now we must think of a sort of segmentation 
also in each metamere, one below the other (Fig. 268). In fact, this we 
must do if we are to understand that which follows. 

Figure 268 shows a combination transverse and longitudinal 
arrangement of metameres with the mesoderm divided into an outer 
(somatopleure) and an inner (splanchnopleure) layer, and the segments 
also divided horizontally. 

The more dorsal portion of the horizontal divisions is called an 
epimere, the mid-portion a mesomere (which is the beginning of the 
excretory system), and the more ventral portion is known as a hypomere. 
The whole metamere is called a mesomeric somite. 

In vertebrates, as we have seen, segmentation is observable pri- 

Origin of the Mesoderm 


marily in the region of the spinal column. Therefore, in the study of 
vertebrates, such as the chick, we shall find that, while segmentation 
begins along the future spinal region, only the more dorsal portion of 
the mesoderm is segmented, and that only partially. The epimeres 
alone, that is, the paired parts lying at the side of the notochord, are 
truly segmented, though the opening in them, the epicoele, shortly dis- 
appears. The mesomeres with their mesocoeles develop into the 
excretory system, and the hypomeres, which have not segmented, but 
whose opening, the hypocoele, is continuous throughout the entire region 
where there has been any segmentation, is now to be known as the 
coelom, or body cavity, into which the internal organs are to grow. 

A/euKa/ 7~ui,e. 

Alet'i/s to .SotnttTi 
Z^afja/ Sfit'^a/ /Verve 


i^ofTTrecf/'ra '^'isue i 
'■tSo)na top/et/fe 


J_: 'S/!>/f>a' ^afr^'^'o 

Fig. 268. 

Stereogram showing the segmentation of the mesothelium. The dorsal and 
ventral walls o£ the coelom later fuse to form the dorsal and ventral mesenteries. 
Al, alimentary canal; EM, epimere; Fb, forebrain; Hh, hindbrain; M, (under 
Sk.c), myotome; Mb, midbrain; MM, me^omere; sk.c, sclerotome, Sp, splanchnic 
layer of the mesoderm (splanchnopleure). (Modified from Kingsley.) 

It is to be remembered that epimere, mesomere, and hypomere are 
composed of mesoderm only. 

As the mesoderm begins to groAv laterad and ventrad, and while it 
is yet unsplit into an outer and inner layer, the thickened portion lying, 
on each side of the neural groove is called the vertebral plate, and the 
more distal portion, the lateral plate. 

The outer layer of the lateral plate, after it splits into two sheets, 
is called somatic mesoderm (and after connecting with the ectoderm, the 
somatopleure) while the inner layer, the splanchnic mesoderm, connects 
with the entoderm and is known as the splanchnopleure (Fig. 268). 

In the head region, the cells of the vertebral plate scatter and com- 
bine with cells which are continually being budded off from the walls 
of the fore-gut to form the mesenchyme of the head region (Fig. 269). 
It will thus be seen that mesenchyme is made up of a combination of 


Embryology of the Chick 

cells from both mesoderm and entoderm, and even of ectoderm, for, 
scattered cells later join from the ectoderm of the head region. 

The somites begin forming in the region of the more anterior end 
of the primitive streak, the first one to develop remaining the more 
anterior. The first four pair of somites take part in the development 
of the hinder part of the head region of the embryo. 

A further important factor to remember at this point is that seg- 
mentation is fundamental, 
and that consequently any 
structures in the body, 
which show segmentation, 
only follow out some plan of 
the original segmentation. 
This is of value in tracing 
the growth of various body- 
parts, such as muscles, for 
instance, in that the nerve 
supply, which we shall 
shortly see is also of seg- 
mental origin, definitely tells 
us where a muscle springs 
from, because nerves always 
follow muscles, and not vice 


Fibrillas in 




Meienchyjna/ Cell 

Cartilaqe matrn 

Cartila<jC ceil 

-S'. ^ 

^ Z 

Fig. 269. 
Figures showing the differentiation of the supporting 
tissues (after Mall). A, white fibers forming in the 
dermis of a 5 cm. pig embryo; B, elastic fibers forming 
in the syncytium of the umbilical cord from a 7 cm. 
embryo; C, developing cartilage from the occipital bone 
of a 20 mm. pig embryo. Mesenchyma from the head of 
a thirty-six hour chick embryo. 

The somatopleure, 
splanchnopleure, and coelom 
become separated into em- 
bryonic and extra-embryonic 
regions later, although, at 
this early stage of which 
we are writing, they form 
continuous structures 
which extend laterally out 
from the germ wall, and 
anteriorly into the head 

The following structures are developed from the embryonic portion 
vascular organs, 
pericardial cavity, 
pleural cavity, 
peritoneal cavity. 

Origin of the Mesoderm 


From the extra-embryonic portion the following are developed : 
embryonic membranes and appendages, 
extra-embryonic portions of the vascular system, 
extra-embryonic coelom (exocoelom). 
Probably the most understandable method of making much of what 
has been said clear, is to use Professor Reese's method of illustration : 

''An understanding of the way in which the embryo becomes folded 
off from the rest of the egg, may perhaps be obtained in the following 
way: Cut out four circles of cloth, say 75 cm. in diameter, of three 
different colors. Put the two circles that are of the same color together, 
and then put these two circles between the other two. 

"Let these superimposed circles represent a greatly enlarged blas- 
toderm that has been removed from the yolk to which it was originally 

Fig. 270. 

Schematic diagrams showing the extra-embryonic membranes of the chick. 
The egg is cut longitudinally while the embryo (which lies at almost right angles 
to the egg), is cut transversely. A, embryo at about 48 hours, B, same at about 
72 hours; C, same at about five days; D, same at about fourteen days. (After 

attached. The upper layer of cloth will represent the ectoblast, the 
bottom layer will represent the entoblast, and the two similarly colored 
layers in the middle will represent the two layers of the mesoblast after 
their separation. 

"As the yolk takes no actual part in the formation of the embryo 
other than as a supply of the food for the growth of the constantly 
enlarging chick, it may be omitted from our model. 

"Now spread the cloth-blastoderm upon a table and place under 
its center a small object, such as a bottle. If now, the fingers of one 
hand be pushed under one end of the bottle, carrying, of course, the three 

40 Embryology of the Chick 

germ layers with them, we shall have represented the formation of the 
head fold. By pushing under the cloth at the other end of the bottle, 
in the same way, we may represent the formation of the tail fold; and 
in a like manner the lateral folds may be formed. If these folds, the 
head, tail, and lateral be pushed under far enough, they will meet under 
the center of the bottle, and we shall have the bottle, with its surround- 
ing layers of cloth, connected with the rest of the model by only a sort 
of stalk, which is hollow and composed of the three layers of cloth. The 
bottle is used simply to give a solid object around which the folding 
may be more easily done, but we are to consider the space occupied by 
the bottle as an empty space. 

"We have now represented what is sometimes called the embryo- 
sac, or simply the embryo, in contradistinction to the yolk-sac, or simply 
the yolk. The embryo remains connected with the yolk throughout the 
period of incubation by the yolk or somatic-stalk, and as the embryo 
increases in size, the yolk-sac is, by absorption, constantly diminished. 
The space occupied by the bottle, in our model, represents the digestive 
tract of the chick, and is lined, as will be seen by examination of the 
model, by the lower germ layer, or entoblast. The body cavity would 
be difficult to represent in the cloth model, but it can be imagined to 
exist as the narrow space between the two layers of similarly colored 
cloth which we have just called the mesoblast. 

"The formation of the amnion may be represented in our model by 
lifting up with the fingers a small fold of the upper and second layers of 
cloth, and pulling these two layers back over the head end of the embryo, 
this fold will correspond to the head fold of the amnion. Similar folds 
might be lifted up at the posterior end and at the sides of the embryo 
model, to represent the tail and lateral folds of the amnion. The way in 
which these folds fuse together will be explained later." 

The allantois cannot be explained from the model, but can be under- 
stood by studying Figure 270. It arises as a thin-walled pouch from 
the posterior end of the digestive tract, and as it increases in size, it 
extends around the upper side of the embryo, between the inner and 
outer layers of the amnion. 

Both amnion and allantois are thrown ofif at hatching, so take no 
permanent part in the actual embryo. 


(About Twenty-four Hours) 

AS the embryo is already well on its way in development at the 
time the egg is laid, and as it has been shown that the extent of 
development varies considerably on account of the retention of the 
egg in the hen for an extra twelve to sixteen hours if it is not ready for 
laying sufficiently early in the day, the formation of the block-like por- 
tions of mesoderm — the somites — becomes the more accurate measure- 
ment of the ag-e of an embryo. Chicks with the same number of somites 
do not usually vary much among themselves in general, though 
individual parts often do; while chicks, which have been incubated for 
the same number of hours, vary considerably in all parts. 

The twenty-four hour stage (four to six somites), (compare Figs. 
264 and 266), is of great importance, for it is during this very early 
period of the chick's life that the interesting and important differen- 
tiating processes are noted. Up to the time the first four somites form, 
the entire growth of the embryo from Hensen's node cephalad, has been 
a formation of the head-region only. 

There has been some question in the past as to whether or not 
additional somites are formed anterior to the first ones thus laid down. 
Professor Patterson performed an interesting experiment which seems 
to warrant our saying that such is not the case. 

Professor Patterson incubated six eggs up to the one somite forma- 
tion period, and then with the most asceptic precautions, opened the 
eggs and marked the first somite by injuring it with an electric needle, 
or inserting a minute glass pin therein. - The shell was then again closed 
by a small piece of egg-shell, and the eg-gs again incubated for varying 
number of hours before being reopened. No new somites appeared 
anterior to the injured one. 

In the study of whole mounts under the microscope, it must be 
remembered that reflected light, coming up from below the object, shows 
different densities as darker or lighter areas. Any portions of the 
embryo, which have become thickened or folded over, will, therefore, 
appear extremely dark and be thus distinguished from the thinner and 
lighter areas. 

At the end of about twenty-four hours we then have : • 

1. Three definite germ layers. (Fig. 271.) 

2. Four to five somites, forming in the vertebral plates, which vertebral 

plates have separated from the lateral plates. 

3. The mesoderm divided into a somatic and a splanchnic layer. 


Embryology of the Chick 

4. The neural groove almost but not quite closed. 

5. A clearly outlined fore-gut and mid-gut. 

6. Clearly defined head-folds, marking the anterior limit of the embryo. 

7. A definite notochord, extending from the anterior end of the primi- 

tive streak to what is to become the mid-brain. 

8. The pellucid area is more or less pear-shaped, and the vascular area 

is seen as an inner zone of the area opaca. 

9. The primitive streak is rapidly becoming relatively shorter and is 

soon to disappear, the cells of which it is composed probably be- 
coming rearranged to form other structures. 

Transverse section through the primitive streak of a chick with six pairs of 
mesodermal somites (about twenty-four hours), showing the formation of the blood- 
vessels and blood. The section extends from the mid-line, nearly half across the 
area vasculosa. b. Blood cells; c, coelomic spaces; e, empty endothelial tubes; ec, 
ectoderm; en, endoderm; gw, germ wall; i, solid blood island; m, axial mesoderm; 
s, primitive streak; si, vascular sinuses of area vasculosa; so, somatic mesoderm; 
sp, splanchnic mesoderm. (After Ruckert.) 

From the twenty-fourth hour on, the texture of the embryo becomes 
firmer, and, whereas, it is difficult to remove an eighteen-hour embryo 
without tearing. The twenty-four hour embryo can easily be removed. 
All outlines also become clearer. 

The anterior part of the embryonal area has thickened, and is 
slightly lifted above the remaining blastoderm, as shown by the crescent- 
shaped anterior boundary (Fig. 266, B). The embryo grows forward 
over this crescent-shaped fold which thus comes to lie under the embryo 
and forms a little pocket between the embryo and the fold called the 
subcephalic pocket. 

The neural folds now unite in the region of the future mid-brain, 
closing rapidly posteriorly, and slowly anteriorly. The closed portion 
is called the neural tube. The most anterior portion, where the neural 
tube will close, is known as the neuropore, which is the region of what 
is later to become the lamina terminalis. This lamina terminalis is 
usually regarded as the morphologically anterior limit of the brain. 
Topographically, however, this is not the case, for the fore-brain grows 
forward and then bends back downward in front of the fore-gut, the 
whole formation becoming bent like a shepherd's crook, so that the 
morphologically anterior end comes to lie on its antero-ventral aspect. 

The neural folds have a somewhat flattened crest, and these fold 
inward, forming a vertical contact. The neural tube is thus formed by 

Four to Six Somite Stage 43 

the fusion of the lower or inner margins of these surfaces, the upper 
ones again forming a continuous ectoderm, so that the neural tube 
becomes entirely separated from it. The cells lying between this ecto- 
dermal and lower margin, and which have been derived approximately 
from the apices of the neural ridges, become the neural crests (Fig. 272). 
These crests do not fuse in the midline, but remain as a pair of longi- 
tudinal bands along the dorsal-lateral surfaces of the neural tube, and 
are the rudiments of the ganglia of the cranial and spinal nerves. They 
are not uniformly developed, and appear much clearer in some sections 
than in others. 

Already very early in the area opaca considerable modification has 

been going on. The area itself has 
broadened and in both lateral and 
posterior regions it is mottled. This 
mottled portion is due to the form- 
ing of differentiated cell groups, 
forming what are called blood- 
islands (Figs. 262, 264), which are 
the beginning of the vascular sys- 
tem. The portion of the area opaca 
in which these blood-islands occur 
is known as the area vasculosa. This 
area vasculosa begins immediately 
Fig. 272. behind the embryo, but then extends 

Transverse section through the head of a i . 11 „ »^ J ^ «-, + ^«.;^-Ur ^^TUi^^ 

7 day Ammocoetes in the region of the tri- laterally and anteriorly, while 
gennnai ganglion. (After ..« Kupffer.) around its periphery a siuglc definite 

blood vessel forms, known as the sinus terminalis (Fig. 284, C). Beyond 
this sinus terminalis, all the remaining area opaca consists of ectoderm 
and entoderm, and extends around the yolk. It is then called the area 

The compact cell masses forming these blood-islands have been 
formed throughout the deeper portion of the germ wall, becoming cov- 
ered superficially with a deep layer of scattered germ wall cells. This 
superficial layer comes to be known as coelomic "mesoderm." 

This superficial layer and the blood islands become continuous very 
early with the mesoderm of the pellucid area derived from the primitive 
streak (Fig. 273). The blood islands become hollowed out, forming 
lacunae. The cells, which have formed the blood islands, become both 
the blood-vessel walls and the blood cells. The lacunae then anastomose, 
forming a complete network extending to meet the vascular structure of 
the pellucid area and later of the embryo. 

The cellular portion of the germ wall, which remains after the 
coelomic ''mesoderm" and the blood islands have differentiated, forms 
the rudiment of the yolk-sac entoderm, to be described later. 

So soon as the extra-embryonic coelom forms, dividing the meso- 


Embryology of the Chick 

derm into somatic and splanchnic layers, the blood vessels remain asso- 
ciated with the splanchnic layer. 

Just as the somites are beginning to form, blood vessels will be 
found at the margin of the pellucid area, beginning to grow toward the 
embryo. Only the tubular vessels develop in the area pellucida; the 
blood islands, as already noted, develop in the area opaca. This means 
that the cellular, or corpuscular, elements of the blood arise in the pos- 
terior region of the area opaca. 


Fig. 273. E 

Beginning of the vascular system in the chick embryo. A, complete blood- 
island; B and C, beginning of vacuole formation; D, vacuoles becoming confluent 
to form the lumen of the blood vessel, b.hem., primitive red blood cells; /, lumen; 
p, vessel-wall; v, vacuole or lacunae. (After Uskow). E. A portion of the 
vascular net work destined to become the aorta in the chick embryo, gv, primitive 
blood forming cells; Ip, cytoplasmic lamina persisting in the lumen of the blood 
vessel that has formed; mar, blood vessels; p, vessel wall;, point of enlargement 
of network; vac, vacuoles. (From Vialleton.) 

Professor Riickert has worked out the arrangement of the blood 
vessels (Fig. 284). He says the vessels in the area pellucida are formed 
by a rearrangement of small groups of cells in the splanchnic mesoderm 
of the area. First, short sections of tubular vessels are formed, which 
then connect with the more peripheral vessels of the opaque area, thus 
forming a continuous vascular network extending toward the embryo 
and finally reaching it at about the time six pairs of somites are formed. 

Soon after this, vessels appear in the embryo itself, the first being 
the paired dorsal aorta in the body region. These are regarded as 
merely straightened axial margins of the vascular network of the area 
pellucida. They diverge widely posteriorly, passing as the vitelline 
arteries into the general vascular network. Anteriorly they are pro- 
longed forward to the heart region where they connect with a pair of 
vessels differentiated in the mesenchyme of the head. 

It will be remembered that the coelom is really paired, and extends 
downward ventrally on both sides until at a later period it meets in the 

Four to Six Somite Stage 


ventral midline and fuses to form the one cavity which it later becomes 
in all vertebrates. The crescent-shaped line where fore-gut and mid-gut 
meet, is called the anterior intestinal portal (Fig. 266, B). 

It is in the region of the anterior intestinal portal that the coelomic 
chambers on both sides show a marked enlargement. The enlargement 
of each side extends mesiad toward the other, and finally both break 
through into each other ventral to the fore-gut, to form the pericardial 
cavity. These enlarged regions are called the amnio-cardiac vesicles 
(Fig. 274) in their early stages ; however, it is better to remember what 


Fig. 274. 

Ventral views of the head ends of chick embryos. A. Embryo with five pairs 
of somites (about twenty-three hours), B. Embryo with seven pairs of somites 
(about twenty-five hours), a.c.v., Amnio-cardiac vesicle; a. i. p., anterior intestinal 
portal; End'c.s., endocardinal septum; FG., fore-gut; Ht., heart; My'C, myocardium; 
N'ch., notochord; N'ch.T., anterior tip of notochord; n.F., neural fold, op. Ves., 
optic vesicle; p.C, parietal cavity (coelomic); Pr'a., proamnion; s.2, sA, second and 
fourth mesodermal somites; V.o.m., omphalo-mesenteric vein. (From Lillie's 
"Development of the Chick" by permission of Henry Holt & Co., Publishers.) 

they are to become, and think of them as the pericardial region of the 

The splanchnic mesoderm is thickened at the point where it lies 
closely applied to the entoderm at the lateral margins of the portal. It 
is from these thickened areas that the paired primordia of the heart 
will arise later. 

It will thus be noted that the heart develops from the ventral and 
not the dorsal aspect. 

The amnio-cardiac vesicles also become vascularized quite like the 
rest of the pellucid area, and a pair of ventral aortae are formed beneath 
the fore-gut. Immediately posterior to the anterior intestinal portal 
these vessels diverge, passing into the vascular network as the rudiments 
of the vitelline veins. 



(Twenty-four to Thirty-six Hours) 

IT IS well at this point to continue with the vascular system, and to 
^ive a connected account of how the heart and the various blood 
vessels are formed. 
The paired primordia of the heart, already mentioned, grow mesiad 
and fuse to form a thin-walled tube which becomes the endothelial lining 
of the heart (Fig. 275). The muscular walls of the heart are formed by 
the addition of an external layer of mesoderm. This is understood the 
better by noting that the splanchnic mesoderm on each side forms a fold 
around the endothelial rudiment and fuses both dorsally and ventrally 

Fig. 275. 

Cross section of A, through head of 2 day chick embryo in the region 
of the mid brain. B, through posterior region of head at the end of 3 days. 
ao, aorta; aud, otic anlage^ c, heart anlage; ch, notochord; d, fore-gut; ec. 
endocardium; ect, ectoderm; ent, entoderm;, neural crest; h.h., hindbrain; 
m.c, myocardium; m.n., midbrain. (After Marshall.) 

in the midline. For a very short period this fusion remains on the 
dorsal aspect, being called the dorsal mesocardium. The ventral fusion 
forms the ventral mesocardium. The ventral mesocardium breaks away 
almost immediately, the dorsal mesocardium remaining for a longer 
time, but then it also disappears with the exception of the portion at 
the anterior and posterior extremities of the heart. The heart is now 
a short median tube made up of endothelial and rudimentary muscular 
layers, suspended in a cavity, later to be called the pericardial cavity. 
Anteriorly, the heart-tube is continuous with a short pair of vessels 
extending into the head-fold— the ventral aortae already mentioned. 

First Half of Second Day 


Fig. 276. 

Posteriorly the heart-tube is continuous with the vitelline or omphalo- 
mesenteric veins (Fig. 276). 

The two vessels of the heart (Fig. 283) come in contact so as to 
form the letter V, with the point of the V toward the head of the 
embryo. The arms continue fusing until a Y-shaped stem has been 
developed, with the stem toward the head. 

Although the two tubes unite in the manner just mentioned, their 

cavities remain distinct for a short time, the endothelial lining forming 

two distinct cavities until a short time after the muscular walls have 

fused. The muscular walls themselves are not complete on the dorsal 

side for a short time, but as soon as the tubes 

have thoroughly fused, the walls also complete 

their function. 

It is the stem of the Y which forms the true 
heart, the two arms being continuous with the large 
vitelline veins which carry blood to the heart from 
the vascular area. The caudal end of the heart is 
then said to be venous, while the cephalic end is 
known as the arterial end. 

At the thirty hour period the heart is a short 
straight tube attached to the ventral wall of the 
fore-gut or pharynx. The point where the vitelline 
veins diverge is at the hindermost angle of the head- 
fold. As the head-fold is pushed farther and farther 
back, the straight portion of the Y is lengthened, 
but as the tubular heart seems to grow more rap- 
idly than does the place to which it is attached, it 
is bent into a loop, with its convexity toward the 
right side of the embryo. This looping is made 
possible by the fact that the heart has by this time 
lost all connection with the wall of the fore-gut and 
remains attached only at both ends. 
It is even before this period that the heart begins to beat, the pulsa- 
tions beginning at the venous end and passing to the arterial end. In 
fact, the beating began before any muscular differentiation could be 
observed in the heart region. 

The cephalic end of the heart is known as the bulbus arteriosus. 
The bulbus branches immediately into two narrow vessels, the aortic 
arches, one passing upward on each side of the digestive tract to the 
dorsal side of the embryo and then running tailward as the paired dorsal 
aortae (Fig. 277). These vessels lie close to the notochord under the 
somites, and extend as separate vessels almost to the tail, where a larger 
branch than the vessel itself is given off from each. These two large 
branches are the vitelline arteries, which carry the blood from the heart 
back to the vascular area. 



Anterior region of 
day chick embryo, 
optic vesicle; crbl, cere- 
bellum; h, heart; m.h., 
vesicle of midbrain; 
med.ohl., medulla oblon- 
gata; r.m., spinal chord; 
U.S., primitive segment; 
v.h'., primary vesicle of 
the forebrain; v.o.m., 
omphalo-mesenteric vein; 
X, anterior wall of prim- 
itive forebrain which 
later expands into the 
cerebrum. (After von 


Embryology of the Chick 


At twenty-seven hours, the more cephalic end of the neural tube has 
become considerably enlarged as compared with the more caudal por- 
tion. The walls are thicker and the lumen larger. This portion is to 
become the brain proper, and the portion in which the lumen has not 
enlarged becomes the spinal cord. A picture of the brain at this time 
(Figs. 278 and 282) will show three primary vesicles or lumen-enlarge- 

^'^-^<^^//^^/ ?>c^/a/ 

Fig. 277. 

Diagrammatic ventral view of a 35-36 hour chick embryo. Compare with Figtures 
279 and 280. (Modified from Prentiss.) 

ments together with what these three vesicles later become. The most 
anterior of the three primary vesicles is known as the fore-brain or 
prosencephalon. The mid portion is called the mid-brain, or mesen- 
cephalon, while the most posterior vesicle forms the hind-brain, or 
rhombencephalon. The rhombencephalon is continuous with the spinal 

As all further developments of the brain arise from these three 
primary regions, it is of the utmost importance that these primary 
regions be grasped fully. 

First Half of Second Day 




position of 
auditory pit 

Fig. 278. 

Diagrams showing neuromeres in brain region of the neural tube. A, lateral 
view of neuralplate of 24 hour chick embryo. B, dorsal view of bram trom a ^o- 
27 hour (7 somite) chick embryo. C, dorsal view of brain from a 30 hour (.lU 
somite) chick embryo. D, dorsal view of brain from a 36 hour (14 somite) chick 
embryo. £, diagram of the segments (neuromeres, myotomes, etc.) ot the neaa 
in longitudinal section. A, anterior myotome;, a, abducens nerve; b, branchial 
clefts; /, facial nerve; g, glossopharyngeal nerve; h, hypoglossal nerve; /, lens 
surrounded by layers of eye; n, nasal pit with the terminal nerve nearby; o, 
oculumotor nerve; op^, ophthalmicus profundus part of fifth nerve; os'; ophthal- 
micus superficials part of the fifth nerve; ot, otocyst; s, spiracular clett; t, 
trigeminal nerve; ta, truncus arteriosus; tr, trochlearis nerve; I-Vili, neuromeres, 
1-6 myotomes. iA-D. from Patten after Hill, E, from Kingsley after Neal.) 


Embryology of the Chick 

From the lateral walls of the prosencephalon the primary optic 
vesicles push out as a pair of rounded pockets, the lumen of each being 
directly continuous with that of the fore-brain. 

The notochord extends as far as the infundibulum (Fig. 282, A), 
(a depression in the floor of the fore-brain), so that all regions of the 
brain lying anterior to it are called pre-chordal, while the rhomben- 
cephalon, mesencephalon, and the part of the prosencephalon posterior 
to the infundibulum, which lie dorsal to the notochord, are called epi- 

.. ' * ^ Prodmnion 

Ani. NGuroporo 
Pros one ephalon 

Forp - qut 

I^hombGnc ^phalon 

,.|_ Vitelline Plexus 
J- Neural Tube 

Lateral Mesoderm 

Phomboid (Sinus 

Primitive Streak 

Fig. 279. 
33 hour chick embryo (12 somites). 

As has been noted previously, the most anterior region, where the 
neural tube closes, is called the neuropore. The neuropore is still 
open at this time and remains so, although gradually becoming smaller 
until after the thirty-third hour period, but even then, there is a scar-like 

First Half of Second Day 51 

fissure. As we know of no structure arisin^r from the neuropore, it is 
important only as a sort of landmark in descril)ing the location of brain 

At this time the neural tube is closed back as far as the somites, and 
it is of nearly uniform diameter, although, posterior to the last formed 
somites, the neural tube is still open, and the neural folds can be seen 
to diverge on either side of Hensen's node (Fig. 279), leaving an opening 
rhomboidal in shape. This is the rhomboid sinus. 

In lower forms, there is an opening from the neural canal into the 
digestive tract, known as the neurenteric canal, or posterior neuropore, 
at the point where the blastopore does not close until after it is sur- 
rounded by the neural folds. In the chick the primitive pit represents 
this region of the neurenteric canal. 

Shortly after the twenty-seventh hour period, and as soon as the 
caudal end of the chick can be definitely outlined, the primitive streak 
disappears entirely, 


The crescent-shaped margin of the anterior intestinal portal grows 
more and more caudad, first because the margin from each side grows 
toward the midline to fuse with the other side, thus lengthening the fore- 
gut by adding to its floor, and pushing the crescentic margin caudad ; 
and second, all structures anterior to the anterior intestinal portal are 
elongating so rapidly that the portal is bound to lie further and further 
caudad from the cephalic end of the embryo. 

These two processes together cause the space between the sub- 
cephalic pocket and the margin of the anterior intestinal portal to 
become elongated, and it is in this enlarging space that the pericardial 
portions of the coelom extend, and in which the heart comes to lie. 


(Thirty-six to Forty-eight Hours) 

Pro- amnion 
Amnion Head- fold 
Optic Vesicle 

Aortic Arch 



Bulbo-conus Arteriosus 



Auditory Pit 
Mesodermal 5omite 

Omphalomesenteric Vein 
Neural Tube 
Omphalomesenteric Artery 

Lateral Mesoderm 
Unsegmented Mesoderm 

Primitive Plate 


Fig. 280. 

38 to 43 hour chick embryo (15 somites), 
i, infundibulum; VIII, ganglion of eighth cranial nerve; A, atrium; V, ventricle. 

T IS at this period that the caudal end of the embryo becomes 
definitely outlined by the formation of a tail fold as well as lateral 
folds similar to the head fold. 

Second Half of Second Day 


Fig. 281. 
Transverse sections through a 36 to 38 hour chick embryo. A, through forebrain; B, through 
the pharyngeal membrane; C, through hindbrain and auditory placodes; D, through posterior end 
of heart; E, through the intestinal portal; F, just posterior to E\ G, through the fourteenth pair 
of segments; H, through the rhomboidal sinus; /, through Hensen's node; /, through the primitive 
streak; K, medial longitudinal section of a Z^ to 38 hour chick embryo. (This drawing mutt be 
studied very carefully and thoroughly to understand the transverse sections which are cut through 
the levels marked.) 


Embryology of the Chick 


Going on from where we left off in our discussion of the formation 
of the three primary brain vesicles, we find that at this period the neural 
canal has completely closed, even the rhomboidal sinus has fused. The 
primary vesicles have enlarged, and their lines of demarcation have 
become more definite. The fore-brain has grown forward as an unpaired 
vesicle from which the cerebral hemispheres are to develop. The walls 
of the brain itself lie under the ectoblast, while between the walls and 
the ectoblast can be seen a small amount of mesoblast which is to form 
the skull. 

The optic vesicles have become elongated and definite constrictions 

thin roof of myelencephalon 

mcso^netenceohalic fold 

(Sylvian a<)u«(luct) 

location of 
posterior commissure 
tuberculum posterius 
diocoele(^vtntricle HI ) 

foramen of Monro 

(sylvian aqueduct^ 

* ventricle IV) 

auditory vesicle 

spinal cord 

Fig. 282. 

Diagrams of brain of 4-day chick embryo. Dotted lines show arbitrary 
boundaries between vesicles. A, longitudinal; B, right side; C, horizontal sec- 
tion. (From Patten, after V. Kupffer.) 

are formed at their bases so that they now form optic stalks, which bend 
downward and backward. 

The cranial nerves can also be seen developing at this period. 

It is at this time that the first bend or flexure takes place in the 
brain, cephalad to the notochord (Fig. 282). This is the cephalic flexure. 

If the neural plate be examined at the end of the first day, eleven 
enlargements (Fig. 278) will be seen with definite constrictions between 

Second Half of Second Day 


them. These enlargements are known as neuromeres and are really an 
uncompleted segmentation. 

The literature of Embryology is filled with many varying and unsat- 


Fig. 283. 

I. Views to show the posterior displacements of the heart in the chick embryo. 
A, The heart lies ventral to the iirst segment. This is the region where the future 
hindbrain will form. B, The point of bending loop of the ventricle is at the seventh 
cervical segment. C, The bending of the loop of the ventricle is now at the ninth 
thoracic segment. (From Corning after Duval.) 

II. The development of the heart of the chick. A-E, ventral views of the 
heart; A, of a forty-hour embryo; B, of a 2.1 mm. embryo; C, of a 3.0 mm. 
embryo; D, of a 5.0 mm. embryo; E, of a 6.5 mm. embryo. F, Frontal section 
through the heart of a 9 mm. embryo, a, Auricle; b, bulbus; d, roots of dorsal 
aorta; e, median endothelial cushion; i, interventricular groove; la, left auricle; 
le, lateral endothelial cushion; Iv, left ventricle; om, vitelline artery; p, left pul- 
monary artery; ra, right auricle; rv, right ventricle; s, interventricular septum; sa, 
interauricular septum; t, roots of aortic arches; v, ventricle. {A, F, after Hoch- 
stetter; B to E after Greil.) 

56 . Embryology of the Chick 

isfactory theories as to what becomes of each neuromere, but as yet 
nothing can be demonstrated satisfactorily. It is conceivable, however, 
that as in the crayfish, for example, where we assume that each separate 
appendage or pair of appendages bespeak an embryological segment, 
so in vertebrates, where optic vesicles grow from the fore-brain, we may 
assume a fusion of several segments. 

At about thirty-three hours, the floor of the prosencephalon has a 
depression formed in it, which is to become the infundibulum. This is 
an important seat of future development. It must, therefore, be studied 
carefully so that it can be recognized in future work. 

At about thirty-eight hours, the three primary vesicles divide to 
form five vesicles (Fig. 278, 282). 

The prosencephalon divides into telencephalon (end-brain), and 
diencephalon ('twixt-brain) ; the mesencephalon remains undivided ; 
while the rhombencephalon divides into metencephalon (cerebellum and 
pons), and the myelencephalon (medulla oblongata). 

The telencephalon has not yet completely separated from the dien- 
cephalon, but there is a median enlargement, showing where the division 
will take place. 

The two most anterior neuromeres of the original rhombencephalon 
form the metencephalon, and the posterior four neuromeres form the 

At thirty-five hours, the auditory pits begin growing as thickened 
ectoderm, known as auditory placodes, on the dorso-lateral surface 
opposite the most posterior inter-neuromeric construction of the 
myelencephalon. At thirty-eight hours, the general level of the ectoderm 
has become depressed to form a pair of cavities known as auditory pits. 
The pits seem to recede until they become closed vesicles, and separate 
from the superficial ectoderm, although it will not be until later that they 
form a definite connection with the central nervous system. 


At about the time the cephalic flexure begins, there is also the 
beginning of a twisting of the entire embryo (Fig. 280), although at 
this time the twisting is only observable in the head region. The 
bending of the cephalic region doAvnward is, as already stated, called 
"flexion.*' The twisting of the embryo from its ventral aspect to its 
side is known as "torsion." 

As the yolk lies directly beneath the embryo, it can easily be under- 
stood that any bending ventrad would be stopped by the large mass of 
inert yolk beneath, so that, if there is to be any considerable bending at 
all, the entire embryo must turn on its side. This it does in all eggs 
which possess considerable yolk, though it does not necessarily come to 
lie on the same side in all amniotes. The chick turns so that its left side 
lies next to the yolk. 

Torsion begins in the head region and gradually and slowly extends 

Second Half of Second Day 


the full length of the body, so that a whole mount, after torsion is 
completed, shows the embryo lying on its left side with head and tail 
close together, the entire embryo forming from one-half to about three- 
fourths of a circle. 


By the end of the second day, the heart has become still more 
twisted, and is now S-shaped with the venous end above and behind the 
arterial end, so that both ends lie close together with the loop as an 

Fig. 284. 

Diagrams of the circulation in the chick embryo and area 
vasculosa. The vascular network of the area vasculosa is omitted 
for the most part. A. Anterior and central parts of the em- 
bryo and vascular area at about thirty-eight hours ( sixteen pairs 
of somites). Viewed from beneath. B. Median and anterior parts 
of vascular area and embryo at about seventy-two hours (twenty- 
seven pairs of somites). Viewed from beneath. C. The main 
vascular trunks of the fourth day. a. Dorsal aorta; aa, aortic 
arches (first and second in A, second, third and fourth in C); 
ac, anterior cardinal vein; al, allantois; au, auricle; b, bulbus 
arteriosus; dC, ductus Cuvieri; dv, ductus venosus; ec, external 
carotid artery; h, heart; ic, internal carotid artery; la, lateral 
dorsal aorta; Iv, left anterior vitelline vein; p, anterior intestinal 
portal; pc, posterior cardinal vein; pv, posterior vitelline vein; 
rv, right anterior vitelline vein; s, sinus venosus; t, sinus ter- 
minalis; tr, venous trunks of the area vasculosa; v, ventricle; va, 
vitelline artery; vv. vitelline or omphalomesenteric vein. (From 
Kellicott after PopofF and Lillie.) 

58 Embryology of the Chick 

intermediate portion between. The venous portion forms a swelling 
which later becomes the auricles, while the arterial end also enlarges 
to form the bulbus arteriosus. The point of the loop becomes the ven- 
tricles (Fig. 283). 

It is toward the end of the second day that the pair of aortic arches 
which have bent dorsad (and continue separately as the paired dorsal 
aortae) unite behind the head to form a single vessel which comes to 
lie directly beneath the notochord. However, after running but a short 
distance caudad, the single aorta again divides into two vessels from 
which the large vitelline artery, already mentioned, is given off on each 
side. The dorsal aortae, now greatly diminished in diameter, continue 
into the tail. 

The first pair of aortic arches formed are called the mandibular 
aortic arches (Fig. 284). 

A second pair now form behind the first, and before the close of 
the day there may be still a third pair, all of which connect in a similar 
manner to the first, with the bulbus arteriosus and the dorsal aorta. 

The sinus terminalis is now also much better developed than before, 
and a true circulation has been established, which can carry the yolk- 
food-granules (after these have been converted into usable food) to the 

It is essential that a somewhat detailed knowledge of the circulation 
be obtained. 

The blood is brought to the heart by the vitelline veins (Figs. 277, 
284). The heart then contracts and forces it through the aortic arches 
into the dorsal aorta. Here it passes tailward, a small portion going 
into the tail itself; but the greater part is carried to the vascular area. 
There are two ways in which the blood now gets back into the vitelline 
veins. First, it may pass directly to the veins from the arteries through 
the connecting capillaries; or, second, it may pass into the sinus 
terminalis at a middle point on each side, and then pass forward and 
backward through this large vessel. The greater portion, however, in 
this second method passes forward toward the head from where it is 
returned to the heart through two large parallel vessels. The part which 
passes backward is again distributed to the vascular area, as there are 
no connecting vessels with the tail of the embryo. The vitelline veins 
and arteries run parallel to each other, though the veins lie a little for- 
ward from the arteries. 

In the embryo itself, the cardinal veins are the main afferent ves- 
sels. At thirty-eight hours the anterior cardinals can be seen. These 
are a pair of vessels which return the blood from the head of the 
embryo to the heart. The posterior cardinals are also paired, and return 
blood from the caudal region. 

Both anterior and posterior cardinal veins unite on each side of the 
body to form a short common vessel before entering the heart, the 

Second Half of Second Day 


right and left ducts of Cuvier, or common cardinal veins. These 
Cuvierian ducts then turn ventrad on each side of the fore-gut and enter 
the sinus venosus at the same point the omphalomesenteric veins enter. 
The omphalomesenteric veins [so called because they pass through 
the umbilicus (navel) as umbilical vessels connecting the offspring with 
the mother in the higher forms], are established in the chick from thirty- 
three to thirty-six hours' incubation. They are postero-lateral exten- 
sions of the self-same endocardial tubes which formed the heart. They 
extend laterad to meet the vessels which develop in the vitelline plexus 
outside the embryo, and which extend inward toward the embryo. The 
omphalomesenteric veins (those lying within the embryo) eventually 
become one with the vitelline veins (those lying in the extra-embryonic 
area) and thus establish the afferent vessels of the vitelline circulation. 

A portion of a cross section of a 54 hour chick embryo 
through the solid anlage of the pronephric tubules in the 
region of the beginning of the Wolffian duct. The nephros- 
tomes are just beginning to form,, nephrostome; 
u.n., pronephric ducts; w. Wolffian duct. (After Kolliker.) 

The efferent vessels develop at about forty hours. They have a dual 
origin. The embryonic vessels consist of the branches of the dorsal 
aortae which extend outward, where they meet with the extra-embryonic 
arteries growing toward the embryo to meet with, and become con- 
fluent with, the embryonic efferent vessels, now being known as the 
omphalomesenteric arteries. 

It is at about the thirty-second hour that the heart begins to con- 
tract irregularly, although the maximum rate (150 to 180 per minute), 
is not reached until after one hundred hours of incubation. 


It is well to follow a corpuscle through its entire circulation at this 
time. With the contraction of the heart, the corpuscle will be sent 

60 Embryology of the Chick 

through the ventral aortae, along the dorsal aortae, out through the 
omphalomesenteric arteries to the plexus of vessels on the yolk. 

It will be remembered that various membranes surround the yolk. 
These contain many small vessels which absorb the yolk. As there 
must be an oxygenation of the blood, this vitelline circulation must 
also assist in this function until the allantois, shortly to be described, 
is formed. This aeration can be accomplished on account of the great 
area these membranes cover, which permits a wide field from which to 
draw the oxygen that permeates through the egg shell and the albumen 
surrounding the yolk. 

After the yolk has been absorbed as food-material, and the blood 
has become oxygenated, the blood is collected into the sinus terminalis 
and the vitelline veins. The latter converge toward the embryo from 
all parts of the vascular layer, and empty into the omphalomesenteric 
veins, which return the blood to the heart. 

The blood which has been sent to the various parts of the embryo 
has in the meantime been returned from the head region by the anterior 
cardinals, and from the caudal end by the posterior cardinals, the 
anterior and posterior cardinal of each side having met to form a short 
common cardinal (duct of Cuvier) through which the blood flows into 
the sinus portion of the heart. 

There is, therefore, a mixed circulation in the heart, consisting of 
both embryonic and extra-embryonic blood. The extra-embryonic, of 
course, is the richer in both food and oxygen supply. 


After about ten somites have been formed, the beginnings of the 
excretory system are visible. 

It will be remembered that the mid-region of the partially seg- 
mented mesoderm, known as the mesomere, is to become the excretory 
system. It can be noted first as a solid cord of cells, extending for two 
or three somites (Fig. 285). This will be called the Wolffian Duct as 
soon as a lumen forms. 

During the second half of the second day, this solid rod elongates 
both headward and tailward, the more tailward portion becoming free 
and lying between ectoblast and mesoblast. A lumen appears toward 
its center and extends headward and tailward simultaneously. About 
the beginning of the fourth day the duct definitely opens into the cloaca. 

The Wolffian body also makes its appearance on the second day, but 
it will be better understood if the description is reserved until later. 



IT WILL be remembered that, when the mesoderm splits into a 
somatic and a splanchnic layer, it extends out over the yolk so that 
there is no definite line of demarcation separating embryo from the 
surrounding media. First, the head fold appears, delimiting the embryo 
at the cephalic end, and later the tail-fold and lateral fold do the same for 

Fig. 2U.' 

Aj B, C, D, four stages of development of the embryonic membranes in birds. 
al, allantois; am, amnion, (in Fig. B., this forms folds which give rise to both 
amnion and serosa); am.h., amniotic cavity; d, digestive cavity; do, yolk-sac. 

E. Cross section through entire egg (including shell). all, allantois which 
begins developing at the blunt end of egg; am.h., amniotic cavity; coel.ex., extra- 
embryonic coelom; do.s., yolk-sac showing development of mid-gut — do.s.d.; do.h., 
covering of yolk; l.k., air-chamber; mes.w., extensions of the mesoderm between 
the communicating opening of yolk sac and amniotic cavitj' — The remaining 
portion of the yolk covering {do.h) closes the passage. These mesodermal exten- 
sions as well as the lower tips, at the pointed end of the eg^, close later and thus 
form a closed amniotic cavity,, amniotic cavity which develops from the 
ectoderm with tiny projections on the inner side. It is in this cavity that ^ the 
remaining yolk is found; Communicating passage between amniotic 
cavity and yolk-sac. {A, B, C, D, after Boas, E, after Duval.) 

the caudal and lateral regions. After these folds have bent downward and 
under the embryo, and almost separated the embryo from the yolk, we 
speak of the space between the somatic and splanchnic layers as the 
intra-embryonic coelom and the extra-embryonic coelom, according to 
which portion lies within, and which portion lies outside, the embr}-©. 
The limiting folds, which are continuous with the head fold and 

62 Embryology of the Chick 

extend on each side of the embryo as the lateral folds, form the line of 
demarcation known as the lateral limiting sulci. 

In this chapter we are concerned with the extra-embryonic mem- 
branes which are developed from the various layers in the extra-embry- 
onic region. The membranes themselves are four in number: the yolk- 
sac, the amnion, the serosa, and the allantois (Fig, 270). 


This is the first of the extra-embryonic membranes to appear. It 
must be remembered that, as the splanchnopleure grows outward from 
the embryo, it surrounds the yolk, thus forming the yolk-sac. The yolk 
itself forms the floor of the primitive gut. 

As the underfolding in the head-region separates the head from the 
remaining blastoderm, it grows caudally and forms an entodermal floor 
to the primitive gut, and the part which thus obtains this entodermal 
floor is called the fore-gut. So, too, in the tail region a little later (about 
the third day), the tail folds under the posterior end of the embryo and 
the part which thus obtains an entodermal floor in that region is called 
the hind-gut. The portion between fore-gut and hind-gut is the mid- 
gut, which is, of course, that region where the yolk is still the floor. As 
the fore-gut and hind-gut become larger and extend toward each other, 
the mid-gut occupies less and less area, until there is merely a little duct 
something like the small end of a funnel, the larger end of the funnel 
being the extended splanchnopleure surrounding the yolk. In other 
words, the mid-gut consists only of the opening of the yolk-stalk, which 
latter is made up in turn of the walls of the splanchnopleure drawn 
together at this point. 

As the neck of the yolk-sac is thus constricted, the omphalomesen- 
teric arteries and veins, which extend throughout the region where the 
constriction takes place, have likewise been drawn into the constricted 
area, and pass to and from the embryo through the yolk-stalk, side 
by side. 

The yolk is now covered with a vascular network spreading through- 
out the splanchnopleure of the yolk-sac, so that the entire food supply 
of the embryo comes to lie in a sac with this circulation of its own 
definitely attached to the mid-body region, though as far as we know, 
no yolk granules pass directly into the embryo, all of it being absorbea 
by the vascular network. In older embryos, the yolk-sac even folds 
considerably, so that a still greater expanse of vascular area is estab- 

The white albuminous portion of the egg rapidly loses the water it 
contains, and is absorbed by the extra-embryonic membranes. 

Ultimately (about the nineteenth day) the yolk-sac is completely 
enclosed within the embryo and then rapidly disappears until it is 
entirely gone by the sixth day after hatching. 

Extra-Embryonic Membranes 63 


While the splanchnopleure forms the yolk-sac, it is the somato- 
pleure, lying outside the embryo, from which both amnion and serosa 
are derived. 

At about thirty hours, the first observable portions of the amnion 
appear as a crescentic fold with the concavity toward the head of the 
embryo. This fold must not be confused with the head fold of the chick 
which folds under the embryo. 

The head at this time sinks into the yolk to a slight degree, just as 
the extra-embryonic somatopleure anterior to the head is thrown into 
the head-fold of the amnion. As the embryo grows anteriorly and the 
^somatopleure caudally, the amniotic fold which is thus folded upon itself, 
forms a double-walled cap over the head of the embryo, gradually 
extending more and more caudad. The caudally directed limbs of the 
head-fold of the amnion continue growing posteriorly on each side of 
the embryo, where they are known as lateral amniotic folds. These 
grow dorsad and mesiad, finally meeting in the midline. 

During the third day, the amniotic tail-fold develops and grows 
cephalad to meet the structures just mentioned, thus forming a complete 
envelop for the embryo. The place where the various amniotic folds 
meet is called the amniotic raphe. 

The amnion is now a completed saccular structure filled with a fluid 
in which the embryo is free to move about and change its position. 
In all probability this ability of free movement also prevents adhesions 
and consequent malformations. 

It is to be noted that the manner in which the amniotic folds came 
into existence, has caused the innermost portions to be ectodermal. 
This ectodermal layer is continuous with the ectoderm of the embryo. 

Likewise, the manner of the somatopleure folding. upon itself, as it 
does, causes two walls to cover the embryo. The inner one is the ecto- 
dermal layer just mentioned, and the outer one is known as the serosa. 
There is a sero-amniotic cavity between the two. 

T4ie som.atopleure now extends peripherally until the entire yolk- 
sac, as well as the embryo, is covered with serosa.^ 

The allantois extends between serosa and amnion. 


This structure differs from the amnion and yolk-sac in that it 
develops within the embryo proper, though it does extend out into the 
extra-embryonic region as it develops. 

About the third day, the allantois develops by an outpushing of the 
ventral wall of the hind-gut (entoderm), pushing the splanchnopleure 
ahead of it, so that we may say it is composed of splanchnopleure with 
an entodermal lining. The following day it pushes out of the embryo 

64 Embryology of the Chick 

into the extra-embryonic coelom, the attached end lying caudal to, and 
parallel to, the yolk-stalk. The proximal portion is the allantoic stalk, 
while the extended bladder-like distal portion is the allantoic vesicle. 

It grows very rapidly from the fourth to the tenth day, and extends 
into the sero-amniotic cavity in a flattened manner. Ultimately it 
encompasses the entire embryo and yolk-sac, and in so doing the 
mesodermic layer of the allantois fuses with the layer of mesoderm of 
the serosa which comes to lie in direct contact with it. This means that 
there is thus formed a double layer of mesoderm, the serosal portion 
derived from the somatic mesoderm, and the allantoic portion derived 
from the splanchnic mesoderm. A very rich vascular network now 
develops between these two layers, connected with the vascular circula- 
tion by the allantoic arteries and veins. 

The allantois thus becomes an organ of respiration, as well as of cir- 
culation, to the developing embryo. As the allantois lies just beneath 
the porous shell, a wide area is presented for an exchange of the carbon 
dioxide developed within the embryo and the oxygen from the outer 

In addition to this function, however, the allantois also serves as 
a reservoir for the secretions from the excretory organs of the embryo, 
and likewise takes part in absorbing the albumen. 


The serosa will become a part of the chorioii in the higher forms, 
and consequently, should be clearly understood at this point. The 
allantoic vessels mentioned above and the mesoderm which lies between 
the serosa and amnion, later fuse with the inner layer of the serosa to 
form the chorion, which is the embryo's organ of attachment to the 
uterine wall of the mother. How very important the allantoic circu- 
lation becomes in mammals may be surmised by realizing that there 
is little yolk in mammalian eggs, which, consequently, forces the embryo 
to receive all of its nourishment from the blood of the mother through 
the uterine walls. The allantoic circulation thus performs the function 
of the vitelline circulation also. 



/ ' / 





Fig. 287., 
64-hour chick embryo. (41 somites.) 

IT IS on the third day that more structures make their first appearance 
than on any other single day of the chick's entire embryonic Hfe. 
The blastod^erm itself has increased in size so that it covers almost 
one-half ^^ yolk surface. The white of the ^^% has decreased in 
amount so that the vascular area has been brought closer to the surface 
under the shell, making aeration of the blood easier. 

The sinus terminalis reaches its greatest functional activity during 
this day, and the vitelline veins have been brought in close contact with 
the vitelline arteries by the growth of the embryo. 

The blood, which the vitelline, or omphalomesenteric, arteries bring 
to the sinus terminalis, still flows headward and tailward as before. The 
portion flowing toward the head returns to the embryo through two 
large vessels lying parallel to the long axis of the embryo, but some- 
times there is only one of these — the one emptying into the left vitelline 
vein. Even if the two vessels are present, the left is the larger. 

It is on the third day also that the single posterior vessel, which 

66 Embryology of the Chick 

also empties into the left vitelline vein and carries blood from the 
posterior region of the sinus terminalis, makes its appearance. 

It will be remembered that it is during this day that the torsion of 
the embryo takes place from the head region posteriorly, so that cross 
sections made from the anterior end will show the embryo turned upon 
its left side, while in the posterior region it still lies upon its ventral 

The flexion continues also, so that the mid-brain becomes the most 
antetior region of the embryo. This flexion not only brings the fore- 
brain in close relation to the heart, but brings optic and otic vesicles 
opposite each other. It will be remembered that the eye-pits form in 
the fore-brain and the auditory pits in the medulla oblongata. 


All parts are growing, and have become larger than on the second 
day (Fig. 288). The important new developments are as follows: 

The epiphysis appears as a small evagination in the midline on the 
dorsal surface of the diencephalon. It later becomes the pineal gland. 

Rathke's Pocket (Fig. 301, 1) is an ectodermal invagination which 
has folded in just beneath the infundibulum. This pocket soon loses its 
connection with the outer epithelium and then becomes permanently 
associated with the infundibulum to form the hypophysis or pituitary 


It will be remembered that these were originally broad stalks 
directly continuous with the cavity of the fore-brain. The cavity, or 
lumen, of the optic stalks is then called an optocoele, and the cavity, or 
lumen of the prosencephalon is called the prosocoele. A constriction 
formed earlier is very marked at about fifty-five hours. The distal ends 
have invaginated, forming a double-layered cup. The newly indented 
layer is termed the sensory layer, because it is from this that the sensory 
layer of the retina is to be formed. The underlying layer is called the 
pigment layer, because it is from this that the pigmented layer of the 
retina is to arise. The invaginated cups are often called secondary optic 
vesicles to disting-uish them from the original vesicles before invagina- 
tion. The original vesicles are then known as primitive vesicles. 

The optic cup does not invaginate so as to form an equally rounded 
edge. The invagination begins at the ventral surface and continues 
dorsally and toward the midline, so that at the place where the invagi- 
nation began, there is a region which has no definite lip. The cup, 
therefore, looks more like a cup that has had this portion broken out. 
This lipless region is known as the choroid fissure. 

The invagination continues for the length of the optic stalk, thus 

Development of Third Day 


forming a fissure in the stalk 
along which, and in which 
the optic nerves and blood- 
vessels come to lie. This is 
on the ventral surface of the 
stalk. In the meantime, the 
optocoele has practically 
become obliterated, a very 
small portion alone remain- 
ing between the sensory and 
pigment layers in the optic 
cup. Even these fuse 
shortly, and then the opto- 
coele entirely disappears. 

The eye lens arises 
independently of the optic 
vesicles in the ectoderm, 
close to the vesicle. At 
forty hours the ectoderm in 
this region has thickened. 
The placodes thus formed 
grow toward and into the 
optic cups after themselves 
forming vesicles. The super- 
ficial ectoderm from which 
they arise, soon closes again 
at the point where the lenses 
have arisen, although a very 
small opening may still be 
seen for a short time. 

It is well to call partic- 
ular attention at this point 
to the similarity of the way 
in which the lens of the eye 
and the auditory vesicle 
develop by a thickening of 
ectoderm, then insinking 
and finally completely sep- 
arating from the superficial ectoderm from which it sprang. The lesson 
to be brought home, is that, once these structures have separated from 
the superficial ectoderm, regardless of their original similarity, each fol- 
lows a totally different line of development and differentiation so as to 
become structurally and functionally unlike in the adult condition. This 
original similarity and adult divergence should be noted throughout 
embryological and comparative studies. 

Fig. 288. 
Diagrams showing brain development in vertebrates. 
I>ongitudinal sections. 

I. Before the blastopore closes. 

II. At the time three regions can be seen. 

I [I. At the time five brain regions have formed. 
(Compare with Fig. 281.) 

A, prosencephalon; aa, dividing line between telen- 
cephalon and diencephalon; c, cerebellum; cc, cerebellar 
commissure; ch, (in I and II) dorsal nerve cord; (in 
III) habenular commissure; _ ^n, neurenteric canal; cp, 
posterior commissure; cw, thickening on optic nerve due 
to the crossing of fibers (this is the chiasma) ; D, dien- 
cephalon; dd, line separating diencephalon and mesen- 
cephalon; e, epiphysis; e', paraphysis; ect, ectoderm; ent, 
entoderm; ff, line dividing mesencephalon and metencep- 
halon; /, inf undibulum ; It, lamina terminalis; M, mesen- 
cephalon; Ml, myelencephalon; Ms, spinal chord; Mt, 
metencephalon; np, neuropore; P, prosencephalon; p, pr, 
pn, neuroporic process; pv, ventral brain-fold; R, 
rhombencepalon ; r, thickening of ectoderm which is some- 
times said to be the anlage of an unpaired olfactory 
groove; ro, optfc recess; si, the groove (sulcus intraen- 
cephalicus) which forms the hindermost boundary of the 
midbrain; T, telencephalon; tp, tuberculum posterius 
(After von Kupflfer.) 


Embryology of the Chick 

In the myelencephalic portion of the brain, the neuromeres have lost 
their dorsal constrictions, though they can still be seen on the lateral and 
ventral surfaces, while the whole cord has thickened. This thickening 
constricts the lumen so that it is quite slit-like at this time. The neural 
tube has closed completely at both anterior and posterior ends at this 

It will be remembered that the neural, or medullary, plates have 
formed, and their lateral folds have begun to unite to form the neural 
groove. This union has now been completed. The ectoderm, dorsal to 


Fig. 289. 
Diagrams of sections through the eye o£ the chick embryo at the end of the 
second day. The dorsal margin is toward the top of the page in A and B.. ,A. Eye 
as viewed directly. B. Vertical section through the line x-cf, in A. C. Horizontal 
section through the line :v-3'. i" A. cf. Choroid fissure; cv, cavity of primary 
optic vesicle; ec, superficial ectoderm of head; i, inner or retinal layer of optic 
cup; /, lens; o, outer or pigmented layer of optic cup; ol, opening of lens sac from 
surface of head; pc, posterior chamber of eye; s, optic stalk, continuous with the 
floor and lateral wall of the diencephalon. (From Lillie "Development of the 
Chick," by permission of Henry Holt & Co., Publishers.) 

the groove, has ag'ain become continuous, leaving a slight area between 
neural groove and superficial ectoderm. 

It will also be remembered that there are small groups of cells on 
each side of the midline, lying within this area, which we called neural 
crests, to distinguish them from the neural folds with which they were 
in close connection. 

The two crests lying on each side of the midline fuse for a time, but 
because they began as two separate groups, they again become separate 
in a short time. They also form a sort of column on each side of the 
midline, running along the long axis of the embryo, but soon they 
segment and become the dorsal root ganglia, or sensory ganglia, of the 
spinal nerves (Fig. 290). As the segmented portions of these neural 
crests also extend into the head region, they there give rise to the ganglia 
of the sensory cranial nerves. 


At the period we are describing, the fore-gut has extended from 
the anterior intestinal portal as its posterior limit to the infundibulum 
as its anterior limit. It is divided into a pharyngeal portion, lying 

Development of Third Day 


ventral to the myelencephalon and encircled by aortic arches, and an 
oesophageal portion, lying posterior to the pharyngeal with a much 
smaller lumen than the pharynx. 

At this time there is an outpushing of the ventral portion of the 
pharynx and an inpushing from the ectoderm close to this region, which 
will soon meet and form the mouth-opening. The ectodermal inpushing 

spinal cord - 
Spina! ganglion -- 

Ventral root - 

Mixed spinal nerve _ 
Mj'Otome - 

Sympathetic ganglion -• 

Fig. 290. 

Developing nerve roots in a chick of 4J^ days. 
(After Neumayer.) 

is known as the stomodaeum, and the thin layer of tissue between the 
inpushing and outpushing, which is later to break through to complete 
the mouth-opening, is called the oral plate. (Fig. 301, I, Seessel's 
pocket.) It is this oral plate region in the adult which separates the 
oral cavity from the pharynx. 

The ^fore-gut extends into the head region cephalad to the 
stomodaeum, and this portion is called the pre-oral gut. This pre-oral 
gut, however, disappears shortly after the oral plate breaks through, 
leaving only a small diverticulum which is then called Seessel's pocket. 

The digestive tract has been lying close to the notochord up to this 
time, being separated from the notochord and the aortae by a broad thin 
layer of mesoderm. Now it begins to draw ventrad from this position, 
remaining attached, however, by the mesentery, a constantly narrowing 
band of tissue. 

This mesentery is composed of mesoblast continuous with that 
which surrounds the entoderm of the digestive canal. The mesoblast 
consists of an undifferentiated middle layer (Fig. 291, b), in which blood 


Embryology of the Chick 

£w.-'a 7<a> 


Development of Digestive 

Transverse section 

Transverse section 

Transverse section 

Transverse section 

Transverse section 

Transverse section 

Fig. 291. 


descending colon 10-mm. pig. 

descending colon 14-mm. pig. 

descending colon 20-mm. pig. 

descending colon 25-mm. pig. 

descending colon 31 -mm, pig. 

descending colon 46-mm. pig. 


a., serosa. 

b., undifferentiated middle layer. 

dm., dorsal mesentery. 

cm., inner circular smooth-muscle layer. 

Im., outer longitudinal smooth-muscle layer. 

mt., mesenteric taenia muscle band. 

sp., Meissner's plexus (submucous). 

ap., Auerbach's plexus (intermuscular). 

sm., serosa. 

subm., submucosa. 

p.m., primordial mucosae cells. 

N. B. — Note especially rapid increase in width of epithelial 
tube and the absolute decrease in thickness of mesenchymal wall due 
to tension stresses elicited by the growth of the former. (Eben J. 
Carey in The Anatomical Record, Vol. 19, No. 4.) 

Development of Third Day 


vessels are developed later, and a superficial layer (Fig. 291, a), of epi- 
thelium, continuous v^ith the epithelial lining of the peritoneal cavity. 
The w^ithdrawal of the anterior part of the fore-gut from the notochord 
is slight, as little or no mesentery is developed in that region. 

It is interesting to note here that the oesophagus has its lumen 
closed for almost its entire length during the sixth day, only to reopen 
from the posterior region anteriorly again in about tv^o days by the rapid 
grov^th of the epithelial tube. This latter growls in a circular direction 
on account of the outer pressure. 

The portion of the intestinal tract immediately posterior to the 
oesophagus becomes dilated on this day to form the stomach. This is 
followed posteriorly by a short region recognized as the duodenum, 
because the beginnings of the liver and pancreas can be observed. 
Mesenchymal cells gather about the oesophagus and stomach from w^hich 
their muscular and connective tissue coats v^ill be derived. 

There w^ill be seen a small pitting-in of the ectoderm to meet the 
underlying entoderm, where the anal opening is to appear. However, 
this posterior opening does not open into the digestive tract until about 
the fifteenth day of incubation. The indenture, which is to become the 
anal opening, is called the proctodaeum. 

The digestive tract is almost straight until the sixth day. Then the 
various loops form and the gizzard develops as a thick-walled outgrowth 
from the end of the stomach. 


Two small hollow outgrowths from the 
ventral side of the oesophagus near its anterior 
end are seen on the third day, the oesophagus 
itself becoming constricted at the point of 

These constrictions form two divisions, the 
more dorsal becomes the oesophagus ; and the 
ventral portion, the trachea. At the point where 
oesophagus and tracheae are continuous, the 
glottis will be formed. 

The trachea grows caudad and bifurcates to 
form pairs of lung-buds. These lung-buds extend 
outward into the surrounding mesenchyme lying 
on either side of the midline. The splanchnic 
mesoderm is pushed ahead of the growing lung- 
buds, until it covers them and forms their outer 
investment layer or pleural covering. The ento- 
derm of the intestinal tract, from which the 
trachea evaginated, forms the entire lining of 
trachea, bronchi, and all air-chambers in the 

Fig. 292. 

Ventral view of lungs and 
air-sacs of 12 day chick 
embryo. ai, anterior inter- 
mediate sac; a, abdominal 
sac; c, cervical sac; I, lat- 
eral part of interclavicular 
sac; lu, lung; m, mesial 
part of interclavicular sac; 
oe, oesophagus; p, posterior 
sac; t, trachea. (From 

Kingsley after Locy and 


Embryology of the Chick 

adult lungs. The connective tissue stroma of the lungs, however, is 
derived from the mesenchyme surrounding the lung-buds. 

In the chick, as in all birds, there is a characteristic thin-walled, 
sac-like outgrowth from the hinder edges of the lungs to form the 
air-sacs (Fig. 292). These do not appear until about the eig'hth day. 


The liver arises as a ventral diverticulum from the duodenum. It 
can be seen for a short time on the lip of the anterior intestinal portal, 

growing cephalad toward 
the fork where the omphalo- 
mesentric veins enter the 
sinus venosus. The liver 
grows out as a series of 
cords, pushing the splanch- 
nic mesoderm ahead of it as 
its investing layer. 

The liver evagination, 
as it forms, retains its open- 
ing into the duodenum (Fig. 
293), which later differen- 
tiates somewhat to become 
the common bile duct, the 
hepatic and cystic ducts, as 
well as the gall bladder. 
Cellular cords bud off from 
the diverticulum and be- 
come the hepatic tubules 
which have secretory func- 

As the intestinal portal 
moves caudad when the 
fore-gut lengthens, the 
proximal portions of the 
omphalomesenteric veins 
come tog'ether and fuse in 
midline. The fusion extends 
caudad nearly to the level of 
the yolk-stalk, beyond which 
they still remain separate. 
The liver now surrounds 
the fused portion of „the 
omphalomesenteric veins. 

It will be noticed, there- 
fore, that the yolk materials 

Fig. 293. 

Two upper cuts are diagrams to show the development 
of the liver, pancreas, and hepatic ligaments, d, intestine; 
ect, ectoderm; leb, liver anluge; lig.hep.ent., ligamentum 
hepato-entericum; lig.susp.hep; ^ ventral mesentery or 
ligamentum suspensorum of the liver; mesent. dors., dorsal 
mesentery; pancr. dors, and pancr. ventr., dorsal and 
ventral pancreas. (After Schimkewitsch.) 

Lower cut is a diagram to show the development of 
the liver. Lobule 1 shows the principal parts of the 
gall capillaries; Lobule 2, shows the anastomoses of these 
gall capillaries; in Lobule 3, only the efferent bile capil- 
laries are shown, together with the arterial and venous 
capillaries, a, arteries; b, veins. (After Stohr.) 

Development of Third Day 


must already at this early period pass directly into and through the liver. 
If this is remembered, it w^ill make the adult portal circulation the better 


The pancreas arises as three diverticula from the duodenum at the 
approximate level of the liver diverticulum. There are three pancreatic 
buds : one medial bud, lying dorsal to the duodenum, and a pair of 
ventro-lateral buds. 

The median bud appears at about seventy-two hours, w^hile the two 
ventro-lateral buds can be seen at the end of the fourth day. The 
dorsal bud arises directly opposite the liver, and grows into the dorsal 
mesentery ; while the ventro-lateral buds arise at the point where the 
liver connects with the intestine, so that both the liver duct and the 
ventral pancreatic duct open into the duodenum by a common duct, 
called the ductus choledochus. Cellular cords grow into masses from the 
three buds, fusing into one glandular mass with two ducts remaining, 
although sometimes all three remain. 


This arises as a median diverticulum from the floor of the pharynx 
at the level of the second pair of gill pouches. By the close of the fourth 
day, the solid rod-like diverticulum, lying in a longitudinal position 
under the floor of the pharynx, has become saccular and remains con- 
nected with the point of origin as the thyro-glossal duct opening at the 
root of the tongue. In mammals, there are additional evaginations at the 
lateral region of the fourth gill pouch. By the sixth day, the thyroid 
body in the chick becomes bi-lobed, the lobes sending out cords of tissue 
which become hollowed out to form the regular adult thyroid tissue. 

The gland then shifts backward and 
becomes surrounded with a sheath 
of vascular connective tissue. 

(Fig. 294) 

This organ arises from the pos- 
terior faces of the third and fourth 
gill pouches after the fourth day of 
incubation. While the organ is orig- 
inally epithelial in character, there is 
soon an ingrowth of mesenchyme 
and the thymus then becomes 
chiefly lymphoid in structure. 


Diagrams to show the development of 
the derivatives of the digestive tract in the 
branchial region. A, Anura,_ B, lizard. cd, 
carotid gland; e^-e^, epithelial bodies; Krd. 
Krm, Krv, dorsal, mid, and ventral remains 
of the axial portions of the gill pouches; p, 
postbranchial bodies; Tml^S, Thymus anlage; 
Tr, Thyroid gland; I-V, Gill slits. (After 


Embryology of the Chick 


Different parts of the embryo grow at different rates of speed, and 
while the heart was formed directly under the anterior end of the diges- 
tive tract on the second day, on the third day the heart has shifted its 
position so far posteriorly that there is a distinct space between it and 
the head proper. This space we may call the neck or pharynx. 

It is in this region that the mesoderm has not divided into the two 
layers — the somatic and splanchnic. We, therefore, still have a sort of 
sheet, consisting of the three primitive layers of ectoderm, mesoderm, 
and entoderm, extending outward from the embryo. 

The entodermal lining of this neck region becomes pushed out into 
four narrow pockets (Fig. 295, A), called the visceral, or gill, pouches, 
during the latter part of the second or the early part of the third day. 
These meet with ectodermal depressions formed as furrows which grow 
inward to meet the gill pouches. The thin wall between the outpushings 
and the ectodermal inpushings breaks through in the lower forms, such 
as in fish and amphibia, and there remains open throughout life; but in 
the chick the opening is seen in the first three pairs during the first three 
or four days. It remains open for about two days. These openings, or 
places where openings usually occur, are known variously as visceral 
clefts, gill clefts, or branchial clefts. 

As the neck is considerably curved, these clefts do not lie parallel 
to each other, but converge toward the ventral part of the neck. The 
fourth cleft never opens in the chick. 

Numbering and naming these clefts begins with the most anterior 
and continues caudad. 

A « 

Fig. 295. 

A, Horizontal, and B, longitudinal section through the head region of Ainmo- 
coetes (larval stage of lamprey), ao.b., anterior aortic arch; ao.d., dorsal aorta; 
ao.v., ventral aorta; di, invagination which separates the anlage of the thyroid gland 
from the digestive tract; m, anlage of mouth; thyr, thyroid anlage; 1, ciliated gill 
region which probably becomes the spiracle; 2-8, gill pouches. {A, after Vialleton; 
B, after Dohrn.) 

Development of Third Day 75 

The first cleft is called the hyomandibular cleft, while the remaining 
ones are known respectively as the II, III, and IV, gill clefts. 

Between these clefts, as well as immediately anterior and posterior 
to them, there is a pair of thickened regions, each pair of which meets 
ventrally in the midline and merges with its mate from the opposite side 
of the body. These thicknesses are called visceral arches, gill arches, 
or branchial arches, also numbered from the anterior end, caudad. The 
first is called the mandibular, the second the hyoid, and from here caudad 
the III, IV, and V. 

The hyomandibular cleft lies between the mandibular and hyoid 
folds or arches. 

It is well at this point to anticipate a little what is to become of 
these structures later (Fig. 296). 

All the clefts close with the exception of the hyomandibular. This, 
too, begins closing at the end farthest from the pharyngeal opening, but 
retains the opening into the pharynx. The unclosed end itself becomes 
the tympanic cavity, while the remaining portion of the cleft becomes 
the Eustachian tube. 

The external auditory meatus is formed by a depression in the sur- 
a n.f. 


Fig. 296. 

Head oi a. SYz day chick embryo, a.n.f., lateral nasal process; 
o«jy eye;, bulbus aortae; i.n.f', inner nasal process; k.h.^ 
and k.h.^ mandibular and hyoid arches; max, upper process of the 
mandibular arch;, nasal groove; st.f., frontal process; tr,nas., 
tear-duct running to nasal cavity; ventr., ventricle; v.h., forebrain. 
(After Duval.) «• 

face ectoderm opposite the position of the tympanic cavity. The outer 
end of the closed hyomandibular cleft thus lies between the tympanic 
cavity and the external auditory meatus, the tissue formed by the closure 
of the cleft forming the tympanic membrane. 

The most posterior two gill arches or folds entirely disappear in the 
adult stages of the chick. 

The pair of mandibular arches grow toward each other on the ven- 
tral side and fuse to form the basis of the mandible, or lower jaw. From 

76 Embryology of the Chick 

the dorsal end of each mandibular arch and at their anterior edge, a small 
branch grows downward and forward during the fourth and fifth days, 
such branch, or branches, being called maxillary processes. There is a 
triangular median process growing toward these maxillary processes 
from the front of the head, known as the fronto-nasal process. The 
maxillary processes form the upper jaw or the maxillary bones. The 
maxillary processes do not fuse with each other, but to each side of the 
fronto-nasal process. When this union does not become complete, the 
well-known abnormality of hare-lip results. 

The formation of clefts and arches may be understood the better by 
the following illustration from Professor Reese : 

With the hands in front of the body (the palmar aspect of each 
hand directed mesiad), and pointed downward, ''bring the tips of the 
fingers together, the fingers of each hand being slightly separated. The 
thumbs should, at first, be closely pressed against the forefingers, and 
should be considered as fused with them. If the fingers and hands are 
slightly bent, there will be a space between the two hands that may be 
taken to represent the pharynx of the chick, while the four fingers will 
represent the first four gill arches, and the spaces between the fingers 
will represent the first three gill clefts. The closure of the visceral clefts 
may be represented by bringing the fingers of each hand together. The 
forefingers, which should, in reality, be the only ones which actually 
meet in the midventral line, will represent the mandibular arch, forming 
the lower half of the mouth. The formation of the maxillary arch, by 
processes budded out from the upper ends of the mandibular arch, may 
be represented by separating the thumbs from the forefingers, and point- 
ing them toward each other without letting them come in contact; the 
triangular space between the thumbs, thus held, being fulfilled in the 
imagination by the fronto-nasal process. The angles between the thumbs 
and forefingers will represent the angles of the mouth. Of course, to 
make the comparison more striking, there should be one more finger to 
represent the hindermost arch and cleft, but as the hinder arches and 
clefts form no part of the adult chick, this omission is of little impor- 


As has been stated, there are already present two or three pairs of 
aortic arches by which blood is carried from the bulbus arteriosus 
around the pharynx to the dorsal aorta. It will be noted that the first 
aortic arch lies in the first (mandibular gill arch), the second in the hyoid 
fold, and so on, each bearing a distinct relation to the correspondingly 
numbered gill-fold. 

The heart, which it will be remembered is attached only at the 
cephalic and caudal ends, is growing rapidly and twisting upon itself. 
The venous, or atrial, side is the more stationary. This side, originally, 

Development of the Third Day 


lay caudal to the arterial, or conus, end of the heart, but in the twisting, 
the conus end comes to lie caudal to the sinus, or venous, end, a position 
that the higher vertebrates all retain in the adult stage. In fishes, the 
atrial region of the heart remains caudal to the ventricular portion even 
in the adults. 

The point w^here the two vitelline veins meet to empty into the 
heart becomes pushed farther and farther caudad, so that the two veins 
unite to form a common opening into the heart.. All blood from the 
vascular area to the heart passes through this single common tube, 
though in a short time the right vein will dwindle away and disappear. 
The tube is then an opening for the left vitelline vein only. This com- 

vp I 

Fig. 297. 
Diagrammatic lateral view of the chief embryonic blood-vessels of the chick, 
during the sixth day. a. Auricle; al, allantoic stalk; ao, dorsal aorta; _ c, cceliac 
artery; ca, caudal artery; cl, cloaca; cv, caudal vein; da, ductus arteriosus; dv, 
ductus venous; ec, external carotid artery; ej, external jugular vein; i, intestine; 
ic, internal carotid artery; ij, internal jugular vein; /, liver; m, mesonephros; ma, 
mesenteric artery ;.m7;, mesenteric vein; p, pulmonary artery; pc, posterior cardinal 
vein; pv, pulmonary vein; s, sciatic artery; sc, subclavian artery ;scv, subclavian 
vein; st, yolk-stalk; sv, subcardinal vein; ul, left umbilical artery; ur, right umbilical 
artery; uv, left umbilical vein; v, ventricle; va, vitelline artery; vca, anterior vena 
cava (anterior carciinal vein), vp, posterior vena cava; vv, vitelline vein; y, yolk-sac; 
3, 4, 6, third, fourth, and sixth aortic arches. (From Lillie's "Development of the 
Chick," by permission of Henry Holt & Co., Publishers.) 

mon tube-like entry into the heart is called the meatus venosus; the por- 
tion nearest th^ heart is the sinus venosus: and the portion lying more 
distal, the ductus venosus. 

The dorsal aorta gives off numerous branches supplying various 
portions of the body of the embryo, the blood being returned by two 
large veins 'on each side of the body. That from the anterior part of the 
embryo is carried through the anterior cardinal veins, and that from 
the posterior part of the body is carried by the posterior cardinal 
veins; the anterior and posterior cardinals then unite into a common vein 
before emptying into the sinus venosus, and this common vein is called 
the duct of Cuvier. 



THE somites have already been described as almost solid triangular 
blocks of cells derived from the dorsal mesoderm. There is a tiny 
opening in the center running horizontally through each somite. 
Oftentimes the opening cannot be seen at all. This opening is called the 

As the embryo continues to increase in size, the triangular block 
becomes more or less circular and there are two layers of cells, an outer 

epithelial layer and an inner 
portion (Fig. 298). The inner 
portion has its cells irregularly 

It is this ill-defined group 
of cells which is known as the 
sclerotome. The cells are 

The sclerotomes of each 
side now grow still farther 
toward the notochord and sur- 
round it. Later they develop 
into the vertebrae. 

The dorsal portion of the 
outer cell mass whose more 
medial portion became the 
sclerotome, has retained its 
definite outlines and epithelial 
characteristics. This portion, 
now called the dermatome, is 
to become the deeper layer of 
the integument. It is important 
to remember at this point that the ectoderm gives rise to the epithelial 
layer of the integument only. 

The portion of the cell mass, w^hich lies medial and slightly ventral 
to the dermatome, is called the myotome. The myocoele now lies 
between the dermatome and the myotome. It is from the myotome that 
the entire skeletal musculature is developed by the ventral walls of the 
myotome, becoming converted into longitudinal muscle fibers. These 
bands of fibres then remain divided into blocks which correspond to the 
original somites. Here again we have a metameric arrangement of 
muscles in the embryo of the chick which corresponds to the segmental 
arrangement of muscles in the adult fish. 

O-afjy //«n 

Fig. 298. 

Diagram of Myotome and Nerve Development. 
The more dorsal portion of the somatopleure is known 
as the dermatome while the dorsal portion of the 
splanchnopleure lying closest to the dermatome forms 
the sclerotome. 

Differentiation of Somites 


The outer portion of the myotome gives rise to the muscles of the 
neck and trunk. The muscles of the appendages arise independently of 
the myotomes. 

T3 germfp %'}:':)] /A/JT- 

Fig. 299. 

The development of the mesonephros. A, B. Transverse sections through the 
mesonephric tubules of the duck embryo with forty-five pairs of somites. After 
Schreiner. C. Transverse section through the middle of the mesonephros of a 
chick of ninety-six hours. From Lillie (Development of the Chick). Ao., Dorsal 
aorta; B., rudiment of Bowman's capsule; c, collecting duct; Cocl., coelom; Col. T., 
collecting tubule; d., dorsal outgrowth of the Wolffian duct; Glom., glomerulus; 
germ, Ep., germinal epithelium; M's't., mesentery; n.t., nephrogenous tissue; r., 
rudiment of conducting portion of primary tubule; T, 1, 2, 3, primary, secondary, 
and tertiary mesonephric tubules; V.c.p., posterior cardinal vein; IV. D., Wolffian 
duct. D, Cross section through the head kidney in the region of the gonads of a 4 
day chick embryo, a, germinal epithelium showing the primary germ-cells c and o; 
a, portion of the peritoneal epithelium which forms the Mullerian duct; E, the 
tissue immediately surrounding the germ cells • which forms the stroma of the" 
gonads later; L, Somatopleure; m, mesentery; WK, Pronephros; .-v. Wolffian duct; z, 
Mullerian duct. (After Waldeyer and O. Hertwig.) 


It is on the third day that the intermediate cell mass — the mesomere 
(Fig. 268, mm) — lying between the somite proper and the point where 
the mesoderm splits into somatic and splanchnic layers, becomes very 
prominent, being covered with sharply defined epithelial cells (Fig. 

It is of great importance for one's future study of embryology as 
well as for the study of comparative anatomy that the development of 
the excretory system be thoroughly understood. 

It is this intermediate cell mass or mesomere, now called the nephro- 
tome, which is to develop into both urinary and reproductive systems. 


Embryology of the Chick 

The Wolffian duct has already been mentioned. The embryonic kidney 
in the chick is called the Wolffian body or mesonephros. This embryonic 
kidney ceases to function very soon after hatching, and is then replaced 
by the metanephros. 

One of the lowest forms of a chordate (an animal which possesses a 

notochord), is the small fish- 
like amphioxus or lanceola- 








tus. In this animal a primi- 

tive form of excretory 

T^^' ^f^ system develops and persists 

t- ;:? B^ throughout the adult life of 

the animal. It is called a 
pronephros, or head kidney. 
This structure develops in 
the frog and other amphibia 
during the embryonic 
period, but it is followed by 
the mesonephros, or Wolf- 
fian body, which becomes 
the permanent kidney of the 
amphibian. In the chick, 
as in all amniotes, the 
mesonephros serves as the 
embryonal kidney, which is 
then followed by the devel- 
opment of a metanephros or 
permanent amniote kidney 
(Fig. 300). 

Notwithstanding the 
type of these three kidneys 
which an animal may pos- 
sess in adult life, all of the 
higher animal forms develop what the immediately lower animal form 
possesses, plus the next succeeding type of pronephros, mesonephros, or 

Amphioxus, therefore, has the pronephros as its permanent kidney; 
amphibians have the pronephros as a sort of embryonic kidney with the 
mesonephros in the adult form ; while all higher types of animals have 
a pronephros (which just appears and degenerates during the early 
embryonic period) with a mesonephros acting as an embryonic organ of 
excretion ; and then, later, from the caudal region of the mesonephric 
duct the adult permanent kidney or metanephros develops. 

To' obtain a clear and accurate view of the functional and structural 
relations of the three kidney-forms, it is important to summarize the 
nephridial theory. 


Schematic arrangement 
metanephros and mesonephros 



show relationship 
in Gymnophiona (trop- 
ical amphibians without tails or legs). II, in advanced 
chick embryo. Ill, one type of its appearance in man. 
I V, in rabbit. The Wolffian duct and ureters are black. 
The canaliculae of the mesonephros are hatched. The 
canaliculae of the metanephros are dotted. (After Felix.) 

Differentiation of Somites 81 

Theoretically, it appears that the waste matter containing nitrogen, 
which is elaborated in the primitive liver and collected in the coelom, 
together with the coelomic fluid itself, passes outward through the 
nephrostomes and tubules in each segment. In higher forms all the 
parts are more differentiated and some of the segmentation is lost. 

Figure 168 (Vol. I) gives a clear understanding of the earthworm's 
segmented excretory system which represents the pronephridic type of 

Such a primitive type of nephridia, if completely developed, may be 
described as follows : At the proximal end of the tubule, a ciUated fun- 
nel, the nephrostome, opens into the coelom. The cilia may continue 
into the tubule to produce a current which will carry the coelomic fluid 
into and through the tubule. The tubule expands into a Malpighian or 
renal corpuscle. This corpuscle consists of a vesicle, known as Bow- 
man's capsule, one side of which projects into the other, so that the 
cavity is nearly filled. This inturned portion is the glomerulus, consist- 
ing of a network of capillary blood vessels, supplied by an artery and 
drained by a vein. Beyond the Malpighian corpuscle the tubule becomes 
convoluted, while its cells become glandular. The first convoluted tubule 
is followed by a straightened portion, forming a simple U shape. The 
arms of the U form the ascending and descending limbs. The entire U 
is called Henle's loop. Then follows a second convoluted tubule which 
passes by means of a short connecting tubule into the non-glandular 
collecting tubule. Other groups of similar-formed excretory units enter 
this same collecting tubule, which then leads into a urinary duct through 
which the waste matter is carried out of the body. 

Various parts of the complete system just described may be miss- 
ing in different groups of animals. For example, in Amniotes, the 
nephrostomes are never formed, though they are formed in Ichthyopsida. 

In the pronephros, the Malpighian corpuscle is quite rudimentary 
and often entirely lacking, and there is also no differentiation into con- 
voluted tubules and Henle's loop. 

The renal corpuscles form a sort of filtering apparatus by which 
water is passed from the blood-vessels of the glomerulus into the tubules 
near their beginning. This liquid thus serves to carry out the urea, uric 
acid, etc., which has been secreted by the glandular portions of the walls 
of the tubules. 

A varying number of nephrotomes are formed in different animal 
forms, and so also a varying number of nephrostomes are formed. Figure 
300 will give the student a general idea of how mesonephros and 
metanephros follow each other and just what their relations are. 

The tiny tubules must not, however, be confused with the ducts. 
The ducts represent the collecting tubule described above. 

The pronephric tubules grow first and then join the pronephric 
ducts. Later the mesonephric tubules grow caudad to the phronephric 

82 Embryology of the Chick 

tubules and join the same ducts. The original pronephric tubules then 
degenerate, so that now the ducts which were originally called pronephric 
become the mesonephric ducts. 

In the real kidney, or metanephros, the tubules do not grow toward 
the mesonephric ducts, but from these ducts. They grow headward and 
laterad and ultimately connect with the tubules of the mesonephros, 
after which the mesonephros itself degenerates with the exception of 
the Wolffian or mesonephric ducts, which in the male become the tubules 
through which the sperm pass. 

With this in mind, the excretory system of the chick can be studied 
with some understanding. 

At about thirty-six hours, it will be remembered,, the pronephric 
tubules were seen to arise from the nephrotome, one pair lateral to each 
somite from the fifth to the sixteenth. Each tubule arises as a solid bud 
of cells with the free ends growing dorsad, close to the posterior cardinal 
veins. The distal end of each tubule is bent caudad later, until it reaches 
the tubule directly posterior to it. Thus is formed a continuous cord of 
cells, which is to become the pronephric duct. These ducts continue to 
extend caudad beyond the region where the tubules were formed, and 
soon develop a lumen. The ducts ultimately reach the cloaca, extending 
ventrally and opening into it. 

The best way to study a series of cross sections is to begin caudad 
and observe them serially toward the head, because the posterior por- 
tions are not so well developed as are the anterior. 

The pronephros (Figs. 285 and 299, D) varies in its development, 
although it usually can be noted in from the fifth to the fifteenth or six- 
teenth somite. Typically it develops from the tenth to fifteenth, inclu- 
sive. No duct is formed anterior to the tenth somite, but the pronephric 
buds in that region disappear by the end of the second day. 

Mesonephric tubules (Fig. 299, A, B, C), develop in all segments 
from the thirteenth or fourteenth to the thirtieth, so that the most 
anterior mesonephric tubules develop in the same segments where the 
pronephric tubules also developed, although it is only posterior to the 
twentieth segment that the mesonephros develops typically. 

The mesonephric tubules, which are to connect with the ducts, are 
developed from radially arranged cell masses lying ventral and medial 
to the ducts. The most anterior of these tubules acquire a lumen by 
the time the ducts have developed their lumen. These tubules grow 
toward and connect with the duct. Later they remain as isolated vesi- 
cles. The grouping of the mesonephric tubules constitutes the 
mesonephros or Wolffian body. Some of the more cephalad mesonephric 
tubules seem to develop nephrostomes opening into the coelom. 

The tubules themselves, having formed separately from the ducts 
and then grown outward and connected with them, have had their out- 
ward ends develop a cluster of closely packed cells which lies in close 

Differentiation of Somites 83 

relationship to the dorsal aorta. This cluster becomes the glomeruli. 
In fact, by the fiftli day, circulation has already been established in the 
glomeruli, and from then until the eleventh day, the mesonephros is at 
the height of its functional activity. Then the metanephros takes its 
place. , Ijii |! l*|jy*| 

The pronephric tubules, which attain even a degree of completeness, 
lie in the tenth to fifteenth somites. It is interesting to observe that it is 
when these tubules begin to degenerate that the glomeruli begin to form 
at the points of the tubules, close to the coelom and actually project into 
the coelom. These bud-like structures are extremely variable, both as to 
number and degree of development. They even develop differently on 
both sides of the chick. They appear to be best developed on the third 
and fourth days. It is for reasons such as these that former writers 
insisted that, in the chick, the pronephros really developed later than the 



UPON opening- an egg which has been incubated for four days, the 
great increase in size of the embryo is the most noticeable feature. 
The germinal mxembrane now covers almost one-half of the yolk, 
and the vascular area is very prominent, althoug-h the sinus terminalis 
has already begun to diminish in distinctness. 

Fig. 301. 

1. Median sagittal section o£ 82 hour chick embryo. 2. Whole mount to show 
regions from which A to O are cut. Sections A=A— A; B=B— B; etc. (Re-drawn 
from Duval.) 

Development of the Fourth Day 


E,lr. E^b. C,.l.^. 


Embryology of the Chick 

The amnion covers the entire chick, but as there is as yet Httle fluid 
in the amniotic cavity, the amnion lies close to the embryo. 

The splanchnic stalk forms a narrow tube, connecting yolk-sac and 
mid-gut, but the somatic stalk has not kept even pace with the splanch- 
nic, so that there is a ring-shaped space between the two through which 
the allantois projects. The allantois is connected by a narrow stalk with 
the hind-gut just cephalad to the tail. 



Fig. 302. 

Appendage muscles being budded off from 
myotomes in the European Dogfish, Pristiurns. b, 
muscle buds; my, myotomes. (From Kingsley 
after Rabl.) 

The cranial flexure increases 
to a considerable extent as does 
also the body flexure, so that the 
embryo now describes a half- 

The muscle plates are nearly 
vertical in position, extending 
almost to the point of separation 
of somatopleure and splanchno- 
pleure, while just beyond this 
point of separation the somatopleure is raised to form a longitudinal 
ridge on each side, which is called the Wolfifian ridge. 

It is on this day also that the beginnings of the appendages, the 
wings and legs, can be seen as local swellings of the Wolfiian ridge. 

These arise (the wing-buds just posterior to the heart region, and 
the hind-limb-buds just anterior to the tail) as conical or triangular 
groups of mesoderm (Fig. 302) covered by ectoderm. By the end of the 
day the wing-buds have become elongated and narrow, while the Hmb- 
buds are short and broad. 

The embryo now lies on its left side, torsion being complete to the 
extent of ninety degrees. 

It is on this day also that a fourth gill cleft appears. The gill arches 
become so thick now that one can scarcely see the aortic arches in any 
of them. 

In the head region, the cephalic flexure presses the ventral surface 
of the head so tightly against the pharynx that the head and pharyngeal 
region must be removed and studied from their ventral aspects or little 
can be observed. 

Figure 296 will show that the mandibular arch forms the more 
caudal boundary of the oral depression, while on each side, the arch 
forms an elevation, the maxillary processes, which grow mesiad and 
form the antero-lateral boundaries of the mouth opening. 

The nasal pits form as hollow depressions in the ectoderm of the 
anterior part of the head overhanging the mouth region with U-shaped 
elevations surrounding them. The median limb is the naso-medial 
process and the lateral limb is the naso-lateral process. The two naso- 
medial processes grow toward the mouth and meet the maxillary 

Development of Fourth Day 


processes which grow inward from each side. It is the fusion of these 
two naso-medial processes with each other in the midhne and with the 
maxillary processes laterally that forms the upper jaw, the maxilla. 

The lower jaw is formed by the fusion in the midline of the right 
and left portions of the mandibular arch. 

Foramen of Monro 
Corpus striatum 

III ventricle 
Ohonoid fissure 

Mesodermal tissue, 
forming later the 
chorioid plexus 




Transverse section through the forebrain of a 16 mm. human 
embryo (six to seven weeks) to show the relationship of the 


Figures 282 and 288 show how the two lateral evaginations of the 
fore-brain stand in relation to the cephalic end of the central nervous 
system, and why it is that the ears come to lie on practically the same 
dorso-ventral level with the eyes, although they begin forming so far 

The development which brings this about has already been dis- 
cussed. Here it is important for the student to observe that the two 
evaginations, forming the telencephalic vesicles, have an open space 
within them, known as the I and II ventricles — also called lateral ven- 
tricles. The portion between them is the III ventricle, which is later to 
become a mere coi;,necting slit-like tube to connect the lateral and more 
posterior ventricles. The entire op^ening in the fore-brain is called the 
telocoele; that in the diencephalon, the diocoele; that of the mesencepha- 
lon, the mesocoele (later called the aqueduct of Sylvius) ; that of the 
metencephalon, the metacoele; and that in the myelencephalon, the 

Figure 282 also shows that what was once the most anterior part 
of the fore-brain, i. e., the lamina terminalis, is no longer so, the lateral 
vesicles having extended further forward. The telencephalic vesicles 
become the cerebral hemispheres in the adult. These become so large 
that they cover the entire diencephalon and mesencephalon. 

All discussion of the central nervous system in our future study of 

88 Embryology of the Chick 

comparative anatomy will depend upon the student's thorough under- 
standing of the development of the brain regions and vesicles as here 
discussed. Consequently, the various arbitrary lines used as demarca- 
tions must be carefully studied. 

The division between telencephalon and diencephalon is the imag- 
inary line drawn from the velum transversum to the recessus opticus. 
The velum is that slight extension marking the point where the primary 
fore-brain is to divide, while the recessus is that transverse furrow in 
the floor of the brain, which leads directly into the lumina of the optic- 

The Diencephalon : There is little change in this on the fourth day, 
except that the infundibular depression in the diencephalon has deep- 
ened, and lies close to Rathke's pocket (Fig. 301, I), with which it later 
fuses to form the hypophysis. Later the lateral walls of the diencepha- 
lon are to become thickened to form the thalami. x\s these thalami 
grow inward toward each other, they will cause the diocoele, or third 
ventricle, to become quite small. The anterior part of the roof of the 
diencephalon remains thin, and the blood-vessels grow downward into 
the diocoele as the choroid plexus. 

The division between diencephalon and mesencephalon is an imag- 
inary line drawn between the tuberculum posterius (a rounded elevation 
in the floor of the brain, of importance only as a landmark of this kind), 
and the internal ridge formed by the original dorsal constriction between 
the primary fore-brain and mid-brain. 

The Mesencephalon : There is little change in this portion of the 
brain, though a little later, dorsal and lateral walls become thickened 
to form either the optic lobes or the corpora quadrigemina. Optic lobes 
and optic vesicles must not be confused, as these are two separate and 
distinct structures. 

The floor of the mesencephalon thickens to form the cerebral pedun- 
cles of the adult, which serve as the main pathway for the fiber tracts 
which connect the cerebral hemispheres with the posterior part of the 
brain and spinal cord. The mesocoele becomes quite small by these 
various thickenings and is now called the aqueduct of Sylvius. 

The Metencephalon : The metencephalon is separated from the 
mesencephalon by the original inter-neuromeric constrictions which 
arose early and marked off this portion of the brain. The caudal boun- 
dary is not well defined, though it is supposed to merge in the myelen- 
cephalon where the roof changes from its thickened state to the thinner 
condition observed more posteriorly. There is little change on the 
fourth day in this region, though later an extensive ingrowth of fiber 
tracts develops both on the ventral and lateral walls. These fiber tracts 
form the pons and the cerebellar peduncles, while the roof of the meten- 
cephalon enlarges to become the cerebellum. 

The Myelencephalon : There is also little change in this region, but 

Development of Fourth Day 8&i 

later the roof becomes thinner, and blood-vessels push their way into 
the opening now called the fourth ventricle, as the posterior choroid 
plexus, while the ventral and side-walls become floor and lateral walls 
of the medulla. 


Along the neural crests already discussed, various ganglia are 
formed. The largest on the fourth day is known as the Gasserian 
ganglion of the fifth cranial nerve. (The fifth is also called the trigemi- 
nal nerve.) It lies ventral and lateral, as well as opposite to the most 
anterior neuromere of the myelencephalon. It forms the sensory nerve 
fibers which grow from the brain mesially and distally into the mouth 
and face region. This fifth cranial nerve is divided into three great 
branches : the ophthalmic, the maxillary, and the mandibular. 

The first branch, the ophthalmic, can be seen on the fourth day 
extending toward the eye, while the other two are just beginning to 
grow toward the mouth angle. 

Just anterior to the auditory vesicle a mass of neural-crest cells is 
developing into what is to become the facial or seventh cranial nerve 
and the acoustic or eighth cranial nerve. This cell mass divides on the 
fourth day to form the geniculate ganglion of the seventh and the 
acoustic ganglion, of the eighth nerve. 

Caudad to the auditory vesicle, the ganglion of the glossopharyngeal 
or ninth cranial nerve can be seen, and the ganglion of the vagus or tenth 
nerve may just be observed. The ninth can be seen in whole mounts, 
the tenth probably cannot. 


Throughout the spinal cord there is a compressed slit-like lumen 
known as the central canal. Just as the ganglia of the cranial nerves 
make their appearance on the fourth day, so, too, do the spinal nerves. 

It requires special methods of staining to study the growth of the 
nerve fibers from fiie neuroblasts, but the development of the spinal 
nerve roots can be studied in ordinarily stained slides. 

It is important to understand that in the adult there will be two 
roots to each spinal nerve (Fig. 290), one ventral, which is motor in 
function, and one dorsal, which is sensory in function. Both of these 
unite lateral to the spinal cord. Immediately distal to this union there 
is a branch extending- to the sympathetic nerve cord. This branch is 
known as the ramus communicans, and extends ventrad. 

Before the union of dorsal and ventral nerve roots takes place, a 
spinal ganglion or dorsal ganglion is seen lying in the dorsal roots. This 
ganglion is formed from the neural crests, and grows toward the cord, 
thus forming the dorsal root, but there are also fibers growing away 

90 Embryology of the Chick 

from the cord from this same gangHonic region which are known as 
peripheral nerves. 

The ventral roots (Fig. 290) are formed by fibers growing out from 
the lateral portions of the cord itself, and are thus efferent nerves carry- 
ing motor impulses from the brain and spinal cord to the muscles. 

The sympathetic ganglia (Figs. 268, 290, 298) arise from cells which 
have migrated ventrally from the neural crests to form cell masses on 
each side of the midline on a level with the dorsal aorta. They are con- 
nected to form cords, and on the fourth day enlargements can be seen 
on these cords opposite the dorsal ganglia. These enlargements are 
the primary sympathetic ganglia, each one of which is connected by a 
ramus communicans to the corresponding spinal nerve. Later, both 
sensory and motor fibers will extend to the sympathetic ganglia from 
the spinal nerve roots as rami communicantes, while fibers running out 
from the sympathetic ganglia connect with the various organs of the 


THE EYE (Fig. 289) 

We have already discussed the projections from the fore-brain 
which are to form the optic cups as well as how the ectoderm directly 
opposite the optic cup thickens to form the lens, this lens then meeting 
with the cup. 

On the fourth day the beginning of almost all the adult structures 
of the eye can be seen. 

The thickened internal layer of the optic cup will give rise to the 
sensory layer of the retina. 

The fibers which arise from the nerve cells in the retina grow along 
the groove in the ventral surface of the optic stalk toward the brain to 
form the optic nerve. 

The external layer of the optic cup will become the pigment layer 
of the retina. 

About the inside of the optic cup a grouping of mesenchymal cells 
can be seen which gives rise to the sclera and the choroid coat. 

Some of the mesenchymal cells even make their way into the optic 
cup through the choroid fissure and give rise to the cellular elements 
of the vitreous body. 

From the margins of the optic cup closest to the lens, the ciliary 
apparatus of the eye is derived. 

From the superficial ectoderm which overlies the eye, the corneal 
and conjunctival epithelium are derived. 

The mesenchymal cells which migrate to the region between the 
lens and the corneal epithelium give rise to the substantia propria of 
the cornea. The lens forms as a thickening of the superficial ectoderm, 
which then becomes depressed so that it forms an invagination into the 

Development of Fourth Day 


optic cup. The margins of the cup narrow and converge toward the 
lens, while the lens itself loses its connection with the superficial ecto- 
derm and forms a completely closed vesicle. A microscopic study of 
sections of; the lens show an elongation of the cells on that side of the 
lens which lies toward the center of the optic cup. These elongating 
cells are to become the lens fibers. 


The auditory placode has already been mentioned as forming on the 
second day. This thickening of the ectoderm sinks below the surrounding 
ectoderm and becomes the floor of the auditory pit. This separates 
from the superficial layer from which it formed. It will be remembered 
that this causes the auditory pit to lie close to the myelencephalon. The 
tubular connection formed by the constriction of the region between the 
sunken placode and the superficial layer where it originally forms, 
remains open for a time as the endolymphatic duct. 

It is by a series of complicated changes that this placode, which 
forms a vesicle, gives rise to the entire epithelial portion of the internal 
ear mechanism. 

Nerve fibers from the acoustic ganglion grow inward to the brain 
and outward to the internal ear, thus forming its nerve connections. The 
external auditory meatus cannot yet be seen, nor has the dorsal and 
inner part of the hyomandibular cleft as yet given rise to the Eustachian 
tube, which is to form later. 

THE NOSE (Fig. 304) 

The olfactory pits are merely paired depressions in the ectoderm 
of the head, ventral to the vesicles of the fore-brain, and just anterior 
to the mouth. These pits become deepened by the growth of the sur- 
rounding parts. The epithelium of the pits ultimately comes to He at 
the extreme upper part of the nasal chambers, and there constitutes the 
sensory epithelium. Nerve fibers grow inward from these cells to the 
lobes of the fore-brain, constituting the olfactory or first cranial nerves. 

Fig. 304. 

Olfactory region of the hen, A in transverse and B in longitudinal section. 
c, middle concha; ch, choana; i, inferior (anterior) concha; o, connection of air 
cavity with head; p, septum of nose; s, superior concha. (From Kingsley after 


Embryology of the Chick 


On the fourth day the mesoderm surrounding the brain has increased 
and begins tO' show slight traces of the skull formation toward the an- 
terior portion of the head, and to extend posteriorly. The fronto-nasal 
process has already been discussed as well as the formation of the upper 
and lower jaws. 

The beginnings of the vertebral column are also in evidence, though 
only tO' a slight extent. Nevertheless, it is well at this point to sum- 
marize what will occur, so that future changes will be understandable. 

During the fourth day the somites have increased from about thirty 
to forty. Each somite now shows a more or less distinctive division into 
an outward lying muscle plate and an inner region which is to form the 
vertebral column. It is from these inner portions that processes of 
mesoderm are sent out by both dorsal and ventral reg'ions to the neural 
canal, as well as below the notochord, until these structures are com- 

L II. 


Fig. 305. 

I, Redividing of the spinal segments. On the left side of the cut the sclerotomes 
and myotomes are seen in their original state. On the right they are seen in their 
final state. The cephalic portions are dotted and the caudal portions hatched. The 
arrows show the line of demarcation between head and neck. II, Ventral view of 
spinal column to show redivided parts of each vertebra. (From Corning, after 

pletely surrounded by mesoderm. By the end of the fourth day these 
processes have become thickened, and are often called the membranous 
vertebral column. The membranous vertebral column is still segmented, 
each segment corresponding to the original somite from which it sprang. 

On the fifth day these lines of segmentation disappear in the meso- 
derm which becomes continuous in its surrounding of the neural canal 
and notochord, though the muscle plates retain their segmentation. 

On the fifth day also, the mesoderm lying in immediate contact 
with the notochord becomes cartilaginous, to form a cartilaginous sheath 

Development of Fourth Day 93 

around the notochord throughout its entire length, while at each side of 
the spinal cord, paired bars of cartilage form, which will shortly fuse 
with the cartilaginous sheath of the notochord to form the beginnings of 
the neural arches. 

Toward the end of the fifth day the points opposite the attachment 
of the neural arches become thickened and more mature, but the por- 
tions between the neural arches retain their embryonic character. This 
causes what has been called a secondary segmentation of the cartilag- 
inous tube. Later this segmentation becomes still greater until the entire 
cartilaginous tube is made up of a series of vertebral rings or segments, 
each segment consisting of a vertebral ring with its attached neural arch, 
and the anterior-posterior halves, respectively, of the succeeding and 
preceding intervertebral rings. Each of these segments becomes one of 
the vertebrae which constitute the spinal column. 

Med. ossif. center 

Fig. 306. 

Thoracic vertebra and ribs of human embryo of 55 mm. 

(Middle of 3rd month) to show ossification centers. Cartilage is 

indicated by stippled areas, and ossification centers by irregular 
black lines. (After Kollmann.) 

It must be understood, however, that these so-called secondary seg- 
ments do not correspond with the somites from which they were formed. 
The secondary lines of segmentation lie at about the center of the muscle 
plates (Fig. 305), so that each of these secondary segments obtains 
approximately one-half of the muscle action from the immediately 
anterior muscle plate, and one-half from the immediately posterior mus- 
cle plate, thus makijig it possible for each one of the vertebrae to have 
the muscles from the two regions act upon it. 

The spinal column develops around the notochord. 

Ossification of the vertebrae begins about the twelfth day in the 
cerLtrum of the second or third cervical vertebra, gradually extending 
caudad. The neural arches ossify still later in two centers of ossification. 
(Fig. 306.) 

On about the seventh day, the centrum of the first cervical vertebra, 
or atlas, separates from the rest of the bony ring and becomes attached 
to the axis to form the odontoid process. 

On the seventh day there are present about forty-five vertebrae. 

94 Embryology of the Chick 

The most posterior five or six fuse a little later and form the pygostyle 
(Fig. 418). 


The anterior tubules of the Wolffian body disappear before the end 
of the fourth day, while the posterior tubules have increased in size and 
become convoluted. The intermediate cell mass from which they arise 
is quite prominent. In cross-sections the convoluted tubules will nat- 
urally be cut at all angles, but they can be distinguished from the duct 
by observing their much thicker walls. The glomeruli can also be seen 
filled with blood vessels. 

The permanent kidney, or metanephros (Fig. 307), begins its 

Fig. 307. 

Diagram of Urogenital Organs. A, in indifferent stage, B, development of 
the male from the indifferent anlagen, and C, development in the female from the 
indifferent anlagen. 

The dotted lines represent the organs in their relative positions in the adult 
stage with the exception of the Mullerian duct in the male and the mesonephric 
duct in the female. These latter ducts disappear for the most part. (After Hertwig.) 

development toward the fourth day in the region lying between the 
Wolffian body and the cloaca, that is, between the thirtieth and thirty- 
fourth segment. 

The metanephric duct, or ureter, forms first, as did the ducts; of the 
pronephros and the mesonephros. This duct grows forward on the outer 
side of the mesoderm lying in the region just mentioned. It grows 
from the dorsal side of the posterior end of the Wolffian duct anteriorly. 
Naturally it has an opening into the Wolffian duct from which it is a 
diverticulum, but on the sixth day it develops a separate opening into 
the cloaca. 

It is from these ureters that lateral outgrowths arise, which join 
with the rods of tissue now forming in the surrounding mesoderm. 
These outgrowths then develop into the tubules and Malpighian bodies 
of the metanephros in a similar manner to the way the Wolffian bodies 

The permanent kidney is quite small when compared with the 
mesonephros, but it increases in size to a considerable extent just before 

Development of Fourth Day 95 


On the fourth day a thickened strip of peritoneum forms on the 
lateral and superior face of the Wolffian body, which later extends all 
the way to the cloaca. This may be called the tubal ridge. It appears 
first at the anterior end of the Wolffian body and grows posteriorly, 
immediately external to the Wolffian duct. This tubal ridge invaginates 
to form a groove-like arrangement at the cephalic end of the Wolffian 
body. The lips of this groove then fuse to form a tube — the MuUerian 
duct. Fusion takes place on the fifth day. The anterior end of this 
Miillerian duct remains open in the coelom. The Miillerian duct becomes 
the oviduct. There are several openings which will develop at the 
anterior end in addition to the main one, but these latter close normally. 
Should they remain open, the abnormal condition of having two open- 
ings in the duct results in the adult stage. The posterior end remains 

The older embryologists considered these two or three openings in 
the Miillerian duct as homologous with the nephrostomes of the 
pronephros, and so insisted that the pronephros followed the 
mesonephros in the chick. Modern embryologists consider that these 
openings lie entirely too far posteriorly and laterally to permit of this 
older interpretation. 

In both sexes so far development has been alike, but on the eighth 
day the Miillerian ducts begin to degenerate in the male. They disappear 
almost entirely by the eleventh day. 

In the female chick, the left Miillerian duct forms the oviduct, while 
the right Miillerian duct degenerates. The left one alone remains 

The Wolffian body disappears almost entirely in the male, though 
a small group of tubules, covering the anterior head of the testes, remains 
as the epididymis. In the female it also disappears almost entirely, the 
part remaining being the parovarium, a small body lying in the mes- 
entery between the ovary and the kidney. 

The Wolffian dyict disappears entirely in the female but acts as the 
vas deferens, or sperm duct, in the male. 

The germ cells probably arise from the entoderm in vertebrates. 
The entoderm is never metameric, though some of the older embryolo- 
gists spoke of metameric gonotomes as primitive segmented regions 
which were to form the gonads. 

At about the time the somites form, the portion of the entoderm 
which is to become the gonads, migrates through the developing meso- 
derm in the epithelium of the genital ridges which have formed imme- 
diately lateral to the mesentery. The primitive or primordial ova, or 
sperm, can be recognized not only from their size but from their reactions 
to microscopic stains (Fig. 254). 

96 Embryology of the Chick 

In the female, the epithelium increases in thickness to an enormous 
extent. The primitive ova multiply, and the products of this multiplica- 
tion, accompanied by some of the epithelial cells, sink into the deeper 
stroma of the connective tissue, and thus form ovarial or medullary 
cords, each such cord containing a number of ova. The cords then break 
up and each egg becomes surrounded by a layer of epithelial cells, the 
whole forming a Graafian follicle. The follicle cells supply the nourish- 
ment to the tgg lying within. 

This whole growth takes place only on the left side of the chick, as 
the right ovary is not functional. 

In the male, the beginnings of the gonad formation are similar to 
that of the female; but instead of the cords breaking up into separate 
follicles, each cord develops a lumen which becomes converted into the 
seminiferous tubule. One can, however, see in the walls of these tubules 
both types of cells that were seen in the Graafian follicle. Indeed, there 
is found a third type of cell called Sertoli's cell, which is supposed to 
act as a sort of nutritive or nurse cell to the developing sperm. 


While these bodies lie closely attached to the kidney, they have not 
developed as a part of the urinary system. 

It is important to know that the adrenal organs, which are among 
the prominent ductless glands now studied in the schools, arise from two 
separate and distinct origins : 

First, by a proliferation of peritoneum, and second, by a prolifera- 
tion of the sympathetic ganglion cells. It is the portion arising from the 
peritoneum which connects with the mesonephros. 

The peritoneal proliferations begin as cords, or strands of cells, along 
the dorsal aorta. These then connect with the renal vesicles of the 
mesonephros. Later, the sympathetic proliferations extend within the 
peritoneal cords, so that the peritoneal cords now become the cortex and 
the sympathetic portions become the medulla of the adult adrenal 


At this point it is well for the student not only to realize, but to 
appreciate the great number of experiments necessary to demonstrate 
biological facts, as well as to understand the great number of possible 
errors and objections which men may bring forth to oppose the inter- 
pretation of these facts after the facts themselves have been demon- 

Suppose the question were raised whether the first beating of the 
heart of an embryo is muscular or nervous in type. What experiments, 
for example, would be necessary to answer such a question satis- 
factorily ? 

Development of Fourth Day 97 

Off hand, one might say that, as nerves carry impulses to all mus- 
cles and as there are ner^^es in the heart muscle, the action must be 

Nerve fibers grow into the heart muscle from the nerve cells close 
by, but the very finest nerve stains known, have been unable to demon- 
strate that there are any nerves whatever in the heart muscle at the time 
of its earliest beating. It may be objected that, however fine our nerve 
stains may be, they are not sufficiently sO' to demonstrate possible nerve 
cells or parts of nerve cells. And that, if we improve in our technical 
ability by obtaining new stains, we may expect to find nerve-cell-sub- 
stance heretofore unseeable. This objection is not well taken because, 
if any muscle be removed from the body and placed in normal salt solu- 
tion, the muscle fibers do not lose their contracting ability, although 
in a few days the nerves degenerate and can be dissected out. If, then, 
our present stains do show the nerve fibers clearly in embryos, and 
these can be seen to be in exactly the same position as in the adult heart, 
as demonstrated by the experiment just cited, it is quite reasonable to 
assume that the stains do show all the nerve fibers that are actually 
present. If this be true, we can demonstrate that all such nerve fibers, 
which normally take a stain, have been destroyed. But, the new nerve- 
less muscle still contracts and expands. 

It could, of course, be argued, in so far as this is embryonic 
material not yet far removed from the germ plasm, that, therefore, every 
particle of the embryonic material still retains some of the undifferen- 
tiated nerve cells, and consequently every part of the embryo does 
actually retain some slight nervous substance which may, under extra- 
ordinary circumstances, be brought forth. 

This objection is overcome by an experiment performed some years 
ago by taking a portion of the adult intestinal tract, chopping it up very 
finely, and placing it in a test tube. Notwithstanding the fact that it 
was thoroughly chopped up, this substance still was able to digest food 
placed in the tube with it. Those who insisted that all action is 
nervous in type, then contended that the different particles of the intes- 
tine still retained soni,e of the essential parts of the nerve cell, so that, 
notwithstanding the fact that the parts had been cut up into very tiny 
particles, the essential nervous elements were still doing the work. 

A portion of intestine was then kept in a chemical medium similar 
to that mentioned in the heart-experiment, and, as with the muscle- 
experiment, the nerves degenerated and were dissected out, although the 
intestine itself continued performing its normal functions. 

If the tough adult nerve-structures are so easily degenerated 
in a normal salt solution, it is surely safe to assume that the hundred- 
fold more delicate embryonic nerve structures will also be destroyed in 
such a medium. 

98 Embryology of the Chick 

It will be remembered that the heart grows as a simple straight 
tube, and that the blood is formed in the blood-islands by the cavities 
flowing together. As these cavities fuse they become tubular, forming 
the vitelline veins which carry the blood to the heart. 

It is of the utmost importance to remember that this early heart 
tube, even before the blood passes through it, has a slow, irregular 
''beat." This, however, is not a true heart-beat but merely the func- 
tional movement of living muscle. 

The true heart-beat is established at that particular moment when 
che thin membrane which separates the anterior from the posterior por- 
tion in the tubular heart breaks through by the greater pressure of the 
blood from the posterior region pressing forward. The tiny membrane 
can be seen to bulge out toward the head region until it finally breaks. 

From that moment on, the blood forces its way through the heart 
and begins a rhythmic muscular reaction on the part of the heart. 

The architecture of all muscles is such that various muscle cells 
are antagonistic to other muscle cells in the same group, so that each 
muscle can, if it elongates, also contract and shorten, the two sets of 
fibers being mutually antagonistic, so as to retain a normal balance. 
The heart muscle shows this principle admirably in that it is composed 
of two groups of spirally wound muscle fibers, the one unwinding as 
the other winds up, thus causing a mutual interaction which keeps up 
by the rhythm of the heart-beat. 

From the study of physics w& know that, when two streams which 
run in different directions meet, a vortex is formed. If we now turn to 
our earlier description of the development of the circulatory system 
during the first two days, we shall find that there are two openings into 
the heart from which streams of blood are brought into that organ. As 
these two blood vessels send their streams together a vortex is formed. 
We thus find a physical explanation as to why the heart muscles follow 
in their growth the optimum stretching caused by the spirally running 
stream of blood. 

From, all that has been said above, it follows that when a heart is 
removed from an animal body and kept "alive" for days or weeks, it 
is but the physical continuation of the normal muscular antagonistic 
reaction of the two spiral shaped groups which have been wound up 
quite as a clock is wound. 

As months and years have elapsed in the winding of these spiral 
muscles, it is quite natural to< understand that they are still sufficiently 
wound when removed from the body so that they will continue in action 
for some days if no external conditions exist to cause a stoppage sooner. 
Such external conditions may be pressure, friction of various kinds, or a 
drying up of the tissues when not retained in proper media. 

If the immediately preceding paragraph be remembered, one can 

Development of Fourth Day 99 

always explain such objections as this: "If potassium is removed from 
the medium in which a heart is placed, it ceases to function, thereby 
proving that it is the potassium solution which causes the reaction." 
It will be remembered that it was stated in the preceding paragraphs 
that the action of the muscles will continue for some time until external 
conditions cause a stoppage. The removal of potassium solution from 
the surrounding medium has nothing to do with the reaction ability in 
the muscle cell itself, but its removal removes a factor necessary to 
reaction, by making the medium one in which it cannot react. An exam- 
ple may make the matter clearer. A living human being has the power 
to move his arms and walk about. This power is retained for many 
years. Let us suppose that we remove certain substances from the air 
which are needed for his lungs to function. An individual breathing 
such an atmosphere would either slowly or rapidly (depending upon 
what gases are removed) grow less and less able to move his arms or 
to walk, and in a short time this ability would cease entirely. In other 
words, such an individual needs a certain kind of atmosphere for breath- 
ing purposes, without which he cannot perform his normal functions. 
This, however, is vastly different from saying that the constituents of 
air are the cause of his being able to move. 

From what has been said above, all that we can say, in regard to 
nerve-and-muscle-action, is that experiments tend to demonstrate that 
muscle cells have the ability to act and react, and that the nerves are 
only the connectors and impulse carriers, by which a coordination of 
muscle cells, which are not in contact with each other, may be brought 

In the embryo, the yolk is converted into blood, and the pressure 
of that blood as it passes through the various vessels with its greater 
posterior and its less anterior pressure, brings about the results men- 
tioned above. In the adult, the food that is taken in and converted into 
blood, works on quite similar principles by continuing to produce a 
greater posterior than an anterior pressure. 

The embryonic circulation can only be understood when it is realized 
that it varies from the adult circulation in a manner that is accounted 
for by the difference between embryonic and adult feeding. In the 
embryo, due to the fact that the food comes entirely from the yolk, there 
is developed a yolk or vitelline-circulation. As the chick's lungs are non- 
functional before birth, and the allantois functions as a respiratory organ, 
there is developed an allantoic circulation, while a third type is the cir- 
culation of the embryo itself. The vitelline and the allantoic together 
constitute the extra-embryonic circulation. 

All food material to the embryo comes from the yolk (although the 
yolk particles do not turn directly into blood. It is the action of the 
entodermal cells which line the yolk-sac and pour out a secretion of 

100 Embryology of the Chick 

enzymes, which breaks down the yolk granules). It is thus seen that it 
is the vitelline vessels which carry food into the embryo, and it is the 
allantois which serves both as a respiratory and excretory organ (at 
least until the nephroi are formed). It is the allantoic circulation which 
permits the escape of carbon dioxide and other waste matters. 

Therefore, the intra-embryonic circulation has nothing to do with 
either manufacturing blood or throwing out waste matter (until the 
nephroi are formed) ; it serves only as the carrier, distributor, and col- 
lecting system of both food and waste materials. 

As all three systems, intra-embryonic, vitelline, and allantoic send 
their vessels to and from the heart, the contents of all three systems 
mingle in that organ, although, of course, the vitelline circulation is the 
richer in food material, and the allantoic the richer in waste matter. 

It is at this point that the student must again remember that arteries 
need not necessarily carry blood rich in food matter, but that an artery 
is any blood-vessel carrying blood away from the heart under a high 
pressure. This pressure probably accounts for the fact that arterial 
walls are thicker and stronger than venous walls. 

Veins are the carriers of blood to the heart. 


This has been described in detail at an earlier period. 


We have already spoken of paired vessels extending through each 
segment of the embryo which arise from the aorta at about the level 
of the allantoic stalk. One pair of these segmental vessels increases in 
size as the allantois grows, and is distributed over the allantois in a rich 
plexus. As the allantois lies close under the shell, there is thus afforded 
a large area where gases can easily be exchanged and oxygenation be 
brought about. After such oxygenation and the extrusion of the carbon 
dioxide, the allantoic blood is gathered by the allantoic veins, and car- 
ried back to the heart. 

The excretory ducts later develop in the embryo and then empty 
into the allantoic stalk close to its cloacal end. It is at this time that 
the allantois begins to function as a receptacle for solid waste matter, 
which, after the fluid parts have been evaporated, retains this waste- 
matter until it is thrown off at the birth of the animal. 

The right and left allantoic veins run cephalad in the lateral body- 
walls of the chick, and enter the sinus venosus, one on each side of ilie 
omphalomesenteric vein. These two allantoic veins will shortly fuse 
and form a single umbilical vein (Fig. 308). 

The yolk-sac is regarded as a diverticulum of the intestine, and the 
allantois as a diverticulum of the urinary bladder, which itself is an out- 
growth of the alimentary tract. 

Development of Fourth Day 



Fig. 308. 

the L? £ At .ho»f .„ V^^°."*/'l^*y ^°";f-. Secondary union of veins around 
F lei;t;,S,'«t,i^ ? ?" °"^ hundred hours. Definite arrangement of the vessels 
F. Relationship of liver vessels, c. Vena cava posterior (inferior); rfC, ductus 

102 Embryology of the Chick 

These outgrowths carry their blood vessels with them. Therefore, 
the omphalomesenteric artery and the vitelHne veins (these latter are 
diverticula of the omphalomesenteric veins) extend out over the yolk, 
constantly increasing as to both absolute numbers and as to branches, 
as the yolk-sac spreads over the yolk. 

The allantoic arteries are also called umbilical arteries. They are 
what will later be known as hypogastric arteries. In birds and reptiles 
five vessels, three arteries (one omphalomesenteric and two allantoic), 
and two veins (one vitelline, really omphalomesentric, and one allan- 
toic), connect the embryo freely through the umbilical stalk (Figs. 284, 
297, 308). 

In mammals, where there is little or no yolk, the yolk-sac is reduced 
or absent entirely arid the omphalomesenteric and vitelline vessels dis- 
appear very early, so that the umbilical cord, or stalk, contains only the 
two allantoic arteries and one allantoic vein. 

In the dogfish and all elasmobranchs, where there is a large yolk-sac 
but no allantois, the vitelline circulation alone is found, the allantoic 
not being present. 


The large vessels communicating with the heart are the first ones to 
appear in the chick embryo. At thirty-three hours the ventral aorta 
extends headward, bifurcating ventral to the pharynx to form a single 
pair of aortic arches. This pair of arches passes dorsad around the 
pharynx and then runs tailward on the dorsal wall of the gut as the 
paired dorsal aortae (Fig. 277). 

On the second day, as the visceral arches and clefts appear, this 
original pair of aortic arches comes to lie in the mandibular arch. In 
each of the visceral arches posterior to the mandibular, new aortic arches 
are formed, which connect the ventral aortae with the dorsal aortae. 

Cuvieri; dv, ductus venosus; g, gut; hi, left hepatic vein; hr, right hepatic vein; 
/, liver; o, omphalo-mesenteric vein; p, anterior intestinal portal; M, rudiment 
of pancreas; ul, left umbilical vein; ur, right umbilical vein; v, vitelline vein; I, 
II, primary and secondary venous rings around the gut. (After Hochstetter.) 
G to /, Diagrams to show the origin of the postcaval vein and the chaneres 
in the abdominal vein in amphibians and reptiles. G, elasmobranch stage. The 
lateral abdominal veins i enter the common cardinal veins c and are not connected 
with the renal portal veins p. H, the lateral abdominals i have joined the renal 
portals at t posteriorly, and anteriorly pass into the liver /, where they unite with 
the hepatic portal vein h; a new vein, the postcaval vein g, is seen growing caudad 
from the liver /, where it arises from the hepatic veins o. I, condition in the 
adults of urodele amphibians; the postcaval vein g, has reached and fused with the 
posterior cardinals e and the subcardinals / at the point r; the two lateral abdominal 
veins have united to form the ventral abdominal vein_ i which empties into the 
hepatic portal h. J, condition in adult reptiles; the anterior portions of the posterior 
cardinal veins n are obliterated, leaving the postcaval vein q as the sole drainage for 
the_ subcardinals / and the kidneys k ; the two lateral abdominal veins remain separate 
as in elasmobranchs. a, anterior cardinal vein; h, sinus venosus; c, common cardinal 
vein; d, subclavian vein; e, posterior cardinal vein; /, liver; ^, _ postcaval^ vein;_/t, 
hepatic portal vein; i, lateral (or in /, ventral) abdominal vein; /, subcardinal vein; 
k, kidney; I, iliac or femoral vein; m, caudal vein; n, obliterated part of the posterior 
cardinals; o, hepatic veins; p, renal portal veins; q, pelvic veins; r, union of 
postcaval, posterior cardinals, the subcardinals; s, union of postcaval and subcardinals; 
t, union of abdominal vein with the renal portal system. (From Hyman's "A 
Laboratory Manual for Comparative Vertebrate Anatomy," by permission of The 
Chicago University Press.) 

Development of Fourth Day 


•At fifty-five hours we saw there were three pairs of these aortic 
arches with a fourth pair just beginning to form. It is at about this 
period also that there is an extension headward from the dorsal aortic 
roots. These extensions form the internal carotid arteries which supply 
the brain. 

The external carotid arteries arise later from the ventral aortic roots. 
They also grow cephalad as do the internal carotid arteries, but, unlike 
the internal carotids, the external carotids supply the face. 

By the end of the fourth day two more pairs of aortic arches appear 

Schematic diagrams illustrating the changes which take place in the aortic 
arches. A, embryonic^ prototype ; B, Fishes; C, Urodeles; D, Lizard; E, Birds; F, 

The dotted lines show the portions which have become obliterated in the adult 
forms of the animals mentioned, ao.asc, ascending aorta which branches into the 
following aortic arches: 0,00, 1, 2, Z, 4; ao.desc, descending aorta; bot, duct of 
Botallus; pulm, pulmonary artery; subcl, subclavian artery; 0,00, 1, 2, 3, and 4, 
the six aortip arches. (After Boas.) 

posterior to those already present. The fifth pair of aortic arches is 
very small and disappears in a short time. 

The first and second arches have become smaller and also finally 
disappear. Probably most often the entire first arch has disappeared 
by this time and sometimes the second has also gone. 

Consequently, there are present only the third, fourth, and sixth 

104 Embryology of the Chick 

pairs. While these arches do not remain intact permanently, though 
parts of them do, it is from these three pairs that the main blood vessels 

In reptiles, birds, and mammals, all the main vessels of the adult 
connecting the heart with the dorsal aorta are derived from the fourth 
pair of embryonic aortic arches. 

It is important to remember this, as our studies in comparative 
anatomy will consist of the study of an amphibian, a dogfish, a turtle, 
and a cat or rabbit, and the student will be required to show similarities 
and differences of this nature in the different groups. 

In reptiles the aortic arches remain in pairs (Fig. 309), but in birds 
the left arch degenerates, while in mammals it is the right arch which 
degenerates. The dorsal aortae, which began as paired vessels, now 
fuse close to the sinus venosus. The portion extending cephalad is 
fused for a very short distance, though never involving the region of 
the aortic arches. 

Quite early in development there are segmental vessels arising from 
the aorta which extend into the dorsal body-wall. The pair at a level 
with the anterior appendage-buds enlarge and extend into th.- wing- 
buds as the subclavian arteries. 

We have already mentioned the pair opposite the allantoic stalk 
which has enlarged to become the allantoic arteries. 

The external iliac arteries which supply the posterior appendage- 
buds arise as branches from the allantoic arteries close to the origin of 
the aorta. 

At four days, the chick embryo still has the omphalomesenteric 
arteries as its main visceral supply. It will be remembered that these 
arteries are paired originally. But as the embryo (which must be con- 
sidered as having its ventral portion open and thus lying extended over 
the yolk of the egg), comes to have its ventral walls meet and grow 
together, the omphalomesenteric arteries, like the heart and other paired 
structures which later become fused to form a single vessel or organ, 
are brought together and fused, thus forming a single vessel which 
comes to lie in the mesentery and runs from the aorta to the yolk-stalk. 

The proximal portion of the omphalomesenteric artery persists as 
the superior mesenteric of the adult, after the atrophy of the yolk-sac. 

The inferior mesenteric artery and the coeliac artery arise from the 
aorta independently at a later stage. 

The cardinal iVeins are the main afferent systems of the early 
embryo. They form on the second day as paired vessels on each side of 
the midline and extend both headward and tailward. The anterior and 
posterior cardinal veins on the same side come together to form the duct 
of Cuvier, which duct runs ventrally and enters the sinus venosus. On 
the fourth day there is practically no change in the cardinal veins. 

Later, the proximal portions of the anterior cardinal veins become 

Development of Fourth Day 105 

connected by a new transverse vessel which forms and enters into the 
venous atrium of the heart, while the distal portions remain as the 
jugular veins of the head region. 

The posterior cardinal veins (Figs. 301, 308) lie in the angle 
between the somites and the lateral mesoderm. It is of importance to 
locate these vessels and understand their position, as the excretory sys- 
tem develops in close relationship to them later, and their relation to 
the excretory system cannot be understood unless their de^'elopmental 
process is closely followed at this stage. 

The mesonephroi develop from the intermediate mesoderm so that 
the posterior cardinal veins lie just dorsal to them throughout their 
length (Fig. 301). 

In fact, the posterior cardinal veins are the principal afferent ves- 
sels of young embryos. However, in the adult these posterior cardinal 
veins will be replaced by the large vena cava. 

With the foregoing in mind as a sort of general view of what has 
taken pla>ce and what will take place in the main blood vessels, we may 
enter into a little more detail. 


The heart began as a paired structure. When the ventral walls of 
the embryo came together, the two portions of the heart also came 
together to form a single tube in the midline of the body, close to the 
ventral portion. 

After this fusion the heart is nearly straight and double-walled. 
The endothelial Hning of the heart has the same structure and is con- 
tinuous with the entering and outgoing blood-vessels. 

There is a thickened layer over the heart, called the epimyocardium, 
which later separates into a thickened muscular layer, the myocardium, 
and a thin non-muscular covering called the epicardium. 

As the paired tubes have come together to form the single heart, 
the splanchnic mesoderm from each side of the body has also come 
together to form the dorsal and ventral mesocardia (Fig. 275). 

The ventral mesocardium disappears almost immediately after its 
formation, but the dorsal mesocardium continues suspending the heart 
for some little tim^, also disappearing ultimately, except at the more 
caudal portion of the heart. 

The heart, now lying in the pericardial cavity, is attached at both 
ends and grows much more rapidly than the surrounding body, so that 
it begins to fold upon itself. The bending of the organ must be care- 
fully studied, or later work upon the heart will have little meaning. 
(Figs. 274, 276, 279, 280, 283, 287.) 

It will be noted that the cephalic end of the heart is attached just 
where the aortae leave it, while the caudal end of the heart is attached 
where the omphalomesenteric veins and the dorsal mesocardium meet. 

106 Embryology of the Chick 

It will also be noticed that the caudal or ventricular end grows 
toward the right. 

The physical restriction placed upon the growing heart by the 
dorsal bending of the entire embryo, and the pushing in of the yolk 
dorsally, plus the fact that the entire embryo (by torsion) comes to lie 
upon its left side, accounts for the particular shape and direction of the 
heart's bending. 

As the U-shaped bend continues to grow, the closed portion of the 
U is forced caudad and twisted upon itself to form a loop. This forces 
the atrial (venous region) portion slightly to the left (that is, toward 
the yolk) and the conus arteriosus is thrown across the atrial region 
by being bent to the right (or away from the yolk), and then caudad. 
The closed portion of the loop is the ventricular region. By this twist- 
ing process the original cephalo-caudal relations of the atrial and ven- 
tricular regions have become reversed, the atrial region now lying 
cephalad to the ventricle. 

Not only has the position of the two regions become reversed, but 
a constriction is formed which divides atrium from ventricle (Fig. 283). 
The constriction itself forms the atrio-ventricular canal. 

It is on the fourth day that the bulbus arteriosus, which lies across 
the atrium, presses down its ventral surface, thus beginning to indicate 
right and left divisions of the atrium. These divisions become complete 

The ventricle has an indication of a right and left division also at 
this same time, caused by a longitudinal groove which appears on its 

The bulbus later divides to form the root of the aorta and the pul- 
monary artery. 

Though the heart began its formation at the level of the hind-brain, 
it has come to lie now on a level with the anterior appendage-buds. 
The ventricular portion is the more unattached and so extends the more 

Histologically, the endocardium of a four-day chick is still a single 
layer of cells, while the myocardium can be distinguished from the outer 
epicardium. The myocardium is composed of elongated cells which 
show some resemblance to the muscle cells they are to form. They 
are arranged in bundles extending toward the lumen. These bundles 
will become the trabeculae carneae of the adult heart. 

The cells of the epimyocardium are becoming flattened to form the 
true epicardium, while loosely placed mesenchymal cells lie in the region 
between endocardium and myocardium near the atrio-ventricular canal. 
These mesenchymal cells will take part at a later period in forming the 
various septa which are to divide the heart into chambers as well as in 
forming the connective tissue frame-work of the valves. 

The ventricular septum is completed at about the sixth day, its 

Development of Fourth Day 107 

anterior edge fusing with the posterior edge of the septum which divides 
the truncus arteriosus into right and left halves. 

The anterior edge of the septum of the truncus arises between the 
fourth and fifth aortic arches in a manner which causes the blood com- 
ing from the left side of the truncus (that is, from the left ventricle) 
to pass through the third and fourth aortic arches, while the blood from 
the right ventricle passes into the fifth aortic arch. 

About the seventh day the right and left parts of the truncus sepa- 
rate completely from each other. The right branch remains connected 
with the fifth aortic arch as the pulmonary trunk, and the left is 
connected with the third and fourth arches as the systemic trunk. 

The ventral ends of the third arches become the subclavian arteries, 
carrying blood to the anterior appendages, while the dorsal communica- 
tion between third and fourth arches disappears. 

This means that the blood now passes from the left side of the heart 
through the third arch to the anterior appendages, and through the 
fourth arch to the dorsal aorta. 

About the fifth day, the fourth pair of arches are the larger of any 
arches remaining, the left one, however, becoming smaller and smaller 
in size until it disappears almost entirely. The right fourth aortic arch 
grows larger and larger to form the systemic arch of the adult chick. 

In mammals it is the right arch which disappears while the left 
alone persists as the systemic arch (Fig. 309). 

Early on the third day, the pulmonary arteries form in the walls of 
the lungs and extend toward the fifth arch with which they connect 
at the ventral ends of these arches. The dorsal end of the fifth arch, 
between the point of union of the pulmonary artery and the dorsal aorta, 
is called the duct of Botallus (Fig. 309). This ductus Botalli offers the 
blood from the right side 'of the heart a passage into the dorsal aorta 
so that little passes through the capillaries. The duct, however, shrivels 
up at the time of hatching, and becomes entirely closed so that all the 
blood from the right side of the heart must pass into the pulmonary 
circulation. It is at this time that the lower portion of the aortic arch 
becomes the pulmonary artery. 


The anterior and posterior cardinal veins unite with each other on 
a side to form the duct of Cuvier and then enter into the meatus venosus. 
These anterior and posterior cardinals bring back the blood to the heart 
from practically all parts of the body except the digestive organs. 

The anterior cardinals persist as the jugular veins to which the 
pectoral veins from the anterior appendages soon become joined. From 
the head and neck the vertebral veins also join the jugulars. 

The posterior cardinals remain large as long as the Wolffian body 

108 Embryology of the Chick 

is functional, but as the permanent kidneys develop, these veins become 
smaller and smaller and ultimately disappear. 

The ducts of Cuvier persist in the adult chick as the anterior venae 

The posterior or inferior vena cava develops from the meatus veno- 
sus, which was formed by the union of the two omphalomesenteric 
veins. To understand the evolving process by which the posterior vena 
cava comes into existence, it is necessary to follow carefully the devel- 
opment of the surrounding organs. 

The liver forms as a diverticulum from the digestive tract. This 
diverticulum then grows around the meatus venosus until it completely 
surrounds the meatus. Blood-vessels form in the liver, extending toward 
the meatus venosus, into which they open by the fifth day. 

At the posterior edge of the liver, there are a number of afferent 
hepatic vessels coming from the meatus venosus through which some 
of the blood coming to the heart from the vascular area may enter the 
capillaries formed in the liver substance. 

At the anterior edge of the liver, where the meatus venosus might 
be said to be leaving the liver, there is a collection of efferent hepatic 
vessels whose distal ends are in direct connection with the capillaries 
of the afferent hepatic vessels. 

The blood passing through the liver has two courses it may take. 
Most of it passes through the large meatus venosus into the heart, but 
some of it passes through the afferent hepatic vessels into the liver sub- 
stance where it is collected by the efferent hepatic vessels and is carried 
to the meatus venosus. 

That part of the meatus venosus lying between the afferent and 
efferent hepatic vessels, is often called the ductus venosus. 

The two allantoic veins already described unite on entering the 
body to form a single vein emptying into the left (persistent) omphalo- 
mesenteric vein. It is well to remember that as the yolk-sac decreases 
in size, the allantois increases, and so, too, the relative size of omphalo- 
mesenteric veins and allantoic veins changes ; the omphalomesenteric 
becomes smaller and the allantoic becomes larger, so that it almost 
seems as though the omphalomesenteric were a branch of the allantoic. 
Both of these veins disappear at the time of hatching. 

The superior mesenteric artery was formed by the closure of the 
ventral body-wall so as to bring the paired omphalomesenteric veins 
together, to form a single vessel running from the aorta to the yolk- 
stalk. As the yolk-sac atrophies, the proximal portion of the omphalo- 
mesenteric artery becomes the superior mesenteric artery. 

The mesenteric vein is formed by a union of the veins from the walls 
of the hinder part of the digestive tract, which there form a single vein. 
This vein is at first quite small, and empties into the omphalomesenteric 
vein just before the latter enters the liver. The point of entry may be 

Development of Fourth Day 109 

said to be the beginning of where the omphalomesenteric vein becomes 
the meatus venosus. 

It will be noted, therefore, that the blood which goes to the liver 
comes from three sources : 

(1) Through the omphalomesenteric vein, from the yolk-sac. This 
blood is rich in food material and has been oxidized in the vascular area. 

(2) Through the allantoic vein from the allantois. This blood is 
very rich in oxygen. 

(3) Through the mesenteric vein from the digestive tract of the 
embryo. This blood is venous in character. 

The mesenteric vein increases in size with the growth of the 
embryo, and after the omphalomesenteric and allantoic veins disappear 
at the time of hatching, it persists as the hepatic portal vein of the adult 
chick. This large vessel brings blood back from the hinder parts of the 
digestive canal to the liver. 

On the fourth day, the posterior, or inferior, vena cava proper arises. 
It forms between the posterior ends of the Wolffian bodies, and runs 
forward in the midline, ventral to the aorta. It joins the meatus venosus 
anteriorly between the heart and the anterior edge of the liver, and pos- 
teriorly it connects with the permanent kidneys as soon as these are 
formed. It also connects posteriorly with the hind limbs and the caudal 

The posterior vena cava is at first quite small, but as more and more 
blood is sent from the developing metanephroi and the caudal region, 
it becomes even larger than the meatus venosus of which it was 
originally but a branch. 

Just before the vena cava becomes larger than the meatus venosus, 
the efferent hepatic vessels have shifted their position so that they now 
enter directly into the vena cava instead of the meatus as formerly. In 
fact, before the time of hatching the entire portion of the meatus venosus 
lying between the heart and liver becomes obliterated, so that all blood 
flowing into the posterior end of the liver through the portal vein, passes 
into the posterior vena cava through the hepatic vein (Fig. 308, I, J). 

The relative changes in the size of blood vessels must be clearly 
understood and followed, or the circulatory system of the embryo, and 
consequently, also the circulation of the adult will be hopelessly 

It is well kt this point to obtain an idea of the embryonic circulation 
of a little later time than that of the fourth day which we have been 

By the beginning of the sixth day, the septa, which have already been 
mentioned, have divided both auricles and ventricles into right and left 
halves (Fig. 283). However, neither of these septa are complete. The 
septum that separates the two parts of the auricle develops perforations, 
and in the human heart these perforations form an oval-shaped opening 

110 Embryology of the Chick 

called the foramen ovale, which may, in the abnormal cases, remain open 
and thus cause a constant intermingling of venous and arterial blood. 
Usually, such persons do not live long, although there are notable 
exceptions. This inter-auricular foramen closes at the time of hatching, 
so that the blood from the right auricle can be sent to the lungs for 
aeration as soon as these organs become functional at birth. 

The septa are sufficiently developed so that we may speak of four 
divisions or cavities in the heart. This makes a double circulation pos- 
sible, namely, the systemic and the pulmonary (up tO' the time of hatch- 
ing, the allantoic circulation takes the place of the pulmonary). 

By this time, then, the heart is fully formed. The sinus venosus has 
been a,bsorbed into the right auricle, of which it forms a part. The open 
foramina allow blood to pass back and forth between the auricles. The 
ventricular septum is more complete. The truncus arteriosus is divided 
into two separate vessels : the pulmonary trunk arises from the right 
ventricle, and the systemic trunk arises from the left ventricle. 

The aortic arches which are still present are the third, fourth, and 
fifth, and small portions of the first and second. 

The systemic trunk from the left ventricle leads to the third and 
fourth pairs of aortic arches, from which the head and fore-limbs are 

The pulmonary trunk, arising from the right ventricle, leads to the 
fifth pair of aortic arches, which are directly continuous with the dorsal 
aorta. It is from these that the small pulmonary arteries arise. 

It will be remembered that, as the lungs are not yet functional, there 
is little use for these vessels until later. An omphalomesenteric artery 
carries blood to the yolk-sac, and a large allantoic artery passes from the 
aorta to the allantois. 

The venous system consists of the right and left anterior venae 
cavae, and the posterior vena cava. The former drain the head and 
fore-limbs, and the latter the posterior portions of the body, the limbs, 
and the kidneys. 

Before reaching the heart, the posterior vena cava is joined by the 
ductus venosus (through which blood is returned from the yolk-sac, 
allantois, and embryonic alimentary canal) by the omphalomesenteric, 
allantoic, and mesenteric veins respectively. 

All three venae cavae open into the right auricle of the heart, but 
due to the position and direction of the opening, and to a valve, the blood 
from the posterior vena cava is directed through the foramen ovale into 
the left auricle, while the blood from the right and left venae cavae 
(anterior) remains in the right auricle. 

As the auricles now contract, the blood which has come from the 
posterior vena cava is forced into the left ventricle and passes out 
through the systemic trunk through the third and fourth pairs of aortic 
arches to the head and fore-limbs, while the blood from the anterior 

Development of Fourth Day 111 

venae cavae passes out through the right ventricle through the pulmon- 
ary trunk and thus through the fifth aortic arches into the dorsal aorta, 
from where the blood goes to the body and hind-limbs of the embryo. 
A small portion, however, is carried out along the omphalomesenteric 
arteries to the yolk-sac and through the allantoic arteries to the allantois 
to take up nutriment and oxygen. In the early embryo, a much greater 
portion of this pulmonary circulation goes to yolk-sac and allantois. 

It is assumed that the vastly greater proportion of blood supply to 
the anterior region, as contrasted with the smaller quantity to the pos- 
terior portions, accounts for the greater and more rapid development of 
the head region, which, it will be remembered, is the first part of the 
chick to develop. 

The disproportionate development of the head may be realized when 
it is known that the human child at birth has a head about one-fourth the 
length of its entire body, while in adults the head extends to only one- 
seventh of the body's length. 

At about the time of hatching, the ductus BotaUi (which it will be 
remembered is that portion of the fifth aortic arch lying between the 
dorsal aorta and the point of origin of the vessel that runs to the lung) — 
(Fig. 309) — closes up entirely, so that the blood from the right ventricle 
must pass through the pulmonary veins back to the left auricle. 

The lungs now become functional and the true pulmonary circula- 
tion is established. The allantoic circulation, which is no longer needed, 
ceases, while the allantoic arteries and veins disappear, as do also the 
omphalomesenteric arteries and veins when the yolk-sac has finished its 
work, and the hatched chick can take in its own food. 

It is at this time also that the entire supply of blood, which goes to 
the liver, passes through the mesenteric vein, which is now called the 
hepatic portal vein,. The ductus venosus has closed, and so all blood 
brought to the liver must pass through the hepatic capillaries before 
reaching the heart. 

The foramen ovale does not close immediately after hatching, but 
does so in a few days. As soon as it does, all blood returned to the 
heart by the three venae cavae is emptied into the right auricle from 
which it is then forced into the right ventricle, thence through the pul- 
monary artery to the lungs, and back through the pulmonary veins to the 
left auricle, from which it is forced into the left ventricle, and finally 
through the systemic trunk. Such an entire separation of venous and 
arterial bloo^ is called a double circulation. 



IN OUR account of the earthworm (Vol. I), the student was introduced 
to all higher forms of animals possessing a coelom or body-cavity. 
The chapter on the earthworm should be reviewed at this point. 

Then, too, in the early part of our work on chick embryology, we 
have seen how the mesoderm divided into splanchnopleure or somato- 
pleure, and how the organs growing out from their respective beginnings 
pushed a layer of one of these coverings before them. And we have also 
seen how the chick embryo is quite similar to an animal which has had 
a ventral incision made along the midline and then had these two halves 
stretched over a yolk-sphere so that its organs or portions of organs, 
which developed from two primordia or beginnings, later came 
together when the fusion of the ventral body walls produced a single 
organ of the two separated halves. 

In adult birds and mammals, the coelom, or body-cavity, consists of 
three regions, known as pericardial, pleural, and peritoneal. The pleural 
region is paired, each half containing one lung. The other two chambers 
are unpaired. The pericardial region contains the heart, and the peri- 
toneal region contains all the abdominal viscera. 

As the coelom arises by a splitting of the mesoderm, and the two 
halves of the chick are spread out over the yolk, the coelom is naturally 
a paired cavity, only becoming a single cavity when the ventral body 
walls of the embryo come together, and the ventral mesentery then 

There are no segmental pouches in the chick coelom as there are in 
some of the lower vertebrates, though it cannot be said that this is unlike 
the lower forms ; for, by the time the coelom appears in the chick, the 
pouches would already be broken through any way, and have become 

As the mesoderm splits and the splanchnopleure and somatopleure 
extend out over nearly the entire yolk-sac, it is to be understood that 
much of this split mesoderm is extra-embryonic. This has already been 
described in an earlier chapter. 

Here we are concerned with the embryonic coelom. 

The portion of the embryonic coelom which gives rise to the three 
body-cavities mentioned above is marked off by a series of folds which 
separate the body of the embryo from the yolk. With the closure of 
the ventral body walls, the embryonic coelom becomes completely sepa- 
rated from the extra-embryonic though in the yolk-stalk region it 
remains open much longer than in other portions (Fig. 281, C to G). 


It is this same closure of the ventral body walls which also brings 
the two portions of the gut together ventrally. This causes the newly- 
closed gut to lie between the two layers of splanchnic mesoderm, while 
the body-spaces on each side form a right and left coelomic chamber. 
In fact, there are double layers of mesoderm which enclose and support 
the gut. These double layered supports are called mesenteries. The 
dorsal mesentery remains as a continuous support — at least the greater 
portion of it does — but the ventral mesentery soon disappears, causing 
the right and left coelomic cavities to become confluent. 

In the liver region, however, the ventral mesentery does not dis- 
appear (Fig. 293). The liver arose by a ventral outgrowth of the gut 
and extended into the ventral mesentery. As the liver grows ventrally 
from the digestive tract there is a portion of the ventral mesentery lying 
dorsal to the liver, that is, between the liver and the gut. This persists 
as the gastro-hepatic omentum while the portion ventral to the liver is 
called the ventral ligament or the falciform ligament. 

The dorsal mesentery persists, as stated, but has different names in 
different parts, i. e., mesocolon, where it supports the colon, mesogaster, 
where it supports the stomach, etc. 

Septa grow out from the body wall to divide the body-cavity into 
the pericardial, pleural, and peritoneal chambers mentioned above. 



ON THIS day the head and tail of the embryo have nearly come 
together by the gTeat curving- of the chick. The yolk is com- 
pletely covered by the blastoderm while nearly two-thirds of the 
blastoderm is vascular area. 


It is during this day that the limb-buds increase considerably in size, 
and are marked off into a proximal rounded portion and an expanded 
distal region. It is in the expanded distal region that the digits can be 
seen to form in cartilage. The rounded proximal portion is slightly bent 
at the points where elbow and knee joints will be formed. 

The elbow and knee-angles at first are directed almost straight out 
from the body, but on about the eighth day both fore and hind-limbs 
rotate until the elbow-joint points caudad, while the knee-joint points 

By the end of the tenth day, both pairs of appendages have their 
definite outlines, though feathers and nails are not yet formed. 

Although the structures which are to become bones are first out- 
lined in cartilage, they later become ossified. There are three well- 
formed digits in the expanded distal portion of the fore-limb at this time 
with a possible fourth in a rudimentary condition, while in the expanded 
distal portion of the hind-limb there are also three well-defined digits 
with two in a rudimentary condition. 

The development of the bony vertebrae has already been discussed. 
Here it is well to state that the ribs develop as cartilaginous bars in the 
body wall of the chick. The ventral ends of these fuse ventrally, and 
after fusion, a portion of each of the fused ends separates from the 
remaining ribs from which they formed. It is this portion which has 
separated that becomes the sternum. 


The skull is divided into two regions: (1) The skull proper, and 
(2) the visceral skull. This latter is that portion of the skull which has 
developed from the visceral arches. 


The notochord forms a sort of central portion around which the 
vertebrae form. The anterior end of the notochord serves a sort of 
similar function in the head region. 

Development of Fifth Day 

On each side of the notochord, a sheet of cartilage develops, 
two sheets are known as parachordal plates (Fig. 310). They 


form a 


o-ch. c/i 







feet s.ot ^^ <kc 

Fig. 310. 

Diaerams of skull formation in Salmon. A, first anlage of cranium. B, C, D, 
successWe^tages in cranial development. Left half of D is ^^ /dvance in 
develooment of right half. au, eye;, base of cranium;, c.b.e.p., 
anierbr and posterior basicapsular commissure (ascending process of palatoquadrate 
cartilage^; c/i, notochord;, trabecular cornu; hyp, opening m which hypophysis 
develoDS- nAr nasal capsule;, and f.b.c, fenestra basicapsularis o.k., ear 
capsule; 'oS;; occipital region parachordal plates;, latera sphenod Tooi oi cranium; tect.s.ot., cartilaginous arch between otic capsules represent- 
ing the cartilaginous roof of higher vertebrates (tectum synoticum) ; fr.,. cran al 
trabeculae. (A, D, after Waskoboynikow ; B, C, from Gaupp after Stohr s model.) 


Embryology of the Chick 

floor for the mid and hind-brains. These plates then fuse both dorsally 
and ventrally around the notochord, and consequently enclose it. The 
fused plate is then known as the basilar plate and forms the floor of the 
hinder portion of the skull. 

The auditory capsules which enclose the auditory organs form and 
fuse to the sides of the basilar plate. It is from growths of the basilar 
plate and the auditory capsules that the floor and occipital portions of 
the skull are formed. 

The anterior portion of the skull is formed from two slender rods 
lying cephalad to the notochord but which are in connection with the 
parachordal plates. These rods are known as trabeculae cranii. 

The pituitary body lies between these trabeculae cranii, so that in 
fusing as they now do to form the ethmoid plate, the pituitary body 

comes to lie in the position where it will 
be found when we study the structure in 
Comparative Anatomy. • 

The ethmoid plate (Fig. 311) extends 
cephalad to the tip of the beak to fuse 
anteriorly with the olfactory capsules. 

The interorbital septum develops as a 
large vertical plate from the dorsal surface 
of the ethmoid plate along the whole median 
line. It is quite common to speak of car- 
tilaginous and membranous bones of the 
skull. This means only that some of the 
bones there formed (in fact, all these we 
have just been describing) were first pre- 
formed in cartilage, and then became bone, 
while the membranous bones were first cartilage, and then, by being 
placed where there was considerable stretching, they became quite thin 
membranes before they finally ossified. 

The membranous bones form the roof of the skull, such, for exam- 
ple, as the parietals, frontals, etc. 


It will be remembered that the first visceral arch was also called the 
mandibular arch, because it is from this arch that the mandible, or lower 
jaw, is formed,^ and that the second visceral arch was known as the hyoid 
arch, because it is from this that the hyoid bone, or cartilage which sup- 
ports the tongue, has developed. The parts of the skull which are thus 
developed from the visceral arches form the visceral skull. 


It is during the fifth day that the interventricular septum is almost 
tompleted, fusing with the posterior edge of the septum which now 

Fig. 311. 

Profile view of 2-day chick-skull. 
as, alisphenoid; d, dentary; e, 
ethmoid; /, frontal; /, prefrontal; 
mx, maxillary; n, nasal; ol, lateral 
occiptal; os, superior occiptal; pa, 
palatine; pm, premaxillary; pt, 
pterygoid; q, quadrate; qj, quad- 
rate- jugal; _ «a, cartilaginous wall of 
nasal cavity; sq, squamosal; st, 
columella; sk, and x., cartilaginous 
portion of skull just becoming con- 
verted into bone. (Cartilage is 
stippled.) (After Boas.) 

^The bones of the upper jaw also form from the mandibular arch. 

Development of Fifth Day 117 

develops in the truncus arteriosus. This latter septum is formed 
between the fourth and fifth pairs of aortic arches and follows a sort of 
spiral course caudally to where it joins the interventricular septum. It 
is the position and shape of these septa which cause the blood to course 
into the different channels as described for the fourth day. 

Two sets of semilunar valves have now formed between the two 
divisions of the truncus arteriosus and the two ventricles into which 
they open. The heart continues growing, but it is not until about the 
twelfth day that the interauricular septum has almost completely closed, 
leaving only the foramen ovale as a small opening between the two 
auricles. The foramen ovale develops a little fold of membrane which 
closes the opening entirely some days after hatching. 

The ventricles now become thickened to a very considerable extent. 
The auricles likewise thicken but not to so great an extent as the ven- 
tricles. The ventricular thickenings on the inside of the heart form as an 
inward growth of ridges which are called trabeculae carneae. They are 
really separate muscle-bundles which help to open and close the valves. 

On the sixth and seventh day the distinctly bird-like characteristics 
appear. Up to this time the beginner cannot tell the difference between 
a chick embryo and that of practically any other one of the higher 

The nasal region now begins to lengthen and the fore-limbs will be 
seen to develop into wings. 

The allantois has become very large and contains a considerable 
amount of fluid. 

The omphalomesenteric arteries and veins now pass from the body 
of the embryo as single vessels. The yolk, though seemingly as large 
as before, is quite liquid in form. 

The flexion of the body is less marked than before, while the head 
is not so large in proportion to the remainder of the body as formerly. 

The cerebral hemispheres can be seen quite plainly, as well as the 
beginnings of the tongue-bud. 

On the next three or four days the little sac-like regions in which 
the feathers develop make their appearance as protrusions from the sur- 
face, especially on the dorsal side of the chick, while a chalky patch at 
the tip of the nose marks the beginning of the horny beak. The yolk 
has become wrinkled and flabby. 

After the eleventh day, the abdominal walls become firmer and the 
intestines are enclosed in the peritoneal cavity. The body is now com- 
plete except for the narrow stalks of the umbilicus and yolk-sac. The 
amniotic fluid tends to disappear, which makes the amnion less 

By the thirteenth day the feathers are well distributed over the 
entire body although they do not break through their sacs until about 

118 Embryology of the Chick 

the nineteenth day. By this time they are approximately an inch in 

On the thirteenth day the nails and scales appear on the toes, and 
by the sixteenth day nails, scales, and beak are firm and well developed. 
On this day also the cartilaginous skeleton completes its growth, and 
various centers of ossification make their appearance. 

By the sixteenth day the white of the egg has disappeared and the 
mesoderm has divided completely into the splanchnopleure and somato- 
pleure all the way around the yolk. 

On the nineteenth day the remains of the yolk are drawn into the 
body cavity of the embryo. 

The embryo begins its development originally by lying with its 
axis transverse to the long axis of the tgg, but by the fourteenth day it 
turns so that its head is toward the air space at the larger end of the 
egg, and at about the twentieth day the chick's beak is pushed through 
the inner covering of the air space so that it can now begin using its 
lungs. It is at this time that the pulmonary circulation begins. The 
blood stops flowing into the umbilical vessels and the allantois conse- 
quently shrivels up and is left inside the shell as the chick pecks its way 



The General Embryology of the Tadpole as Compared 
with that of the Chick 

A S IN our study of the Embryology of the chick, it is essential that 
/\ the student again read the chapters on mitosis, fertilization, and 
Jl \the summary on Embryology, and then go over each system in the 
developing chick corresponding to the system he may be studying in the 
frog. Only in this way can the comparison of the developmental pro- 
cesses be understood. 

After this has been done, the following groups of Craniata must be 
kept clearly in mind to make clear the various embryological relation- 
ships which must be referred to, not only in the study of Embryology, 
but also in Comparative Anatomy. 


(After Newman) 

Sub-Phylum I. Cephalochordata (Adelochorda), (Fig. 312). 
I. II. 

^h'iw ^^<■'-: \^^ v??^c:T^^i? 

Fig. 312. 

I. Examples of Amphioxus (Branchiostoma and Lancelet), Tunicates (First two upper figures), 
Lamprey (The large, lower, left-hand figure-adult; and the embryo lamprey, usually called Ammocoetes-^ 
2 upper right-hand figures), and the Hag fish (2 lower right-hand figures). 

II. Sketch of chief kinds of Urochordata showing distribution in sea. Dotted lines on left 
indicate life-zones. The surface is called the pelagic zone. (From Herdman.) 

This includes but a single family of fish-like creatures, of which 
there are about twelve species. The type form is Amphioxus 
more correctly known as Branchiostoma. 


The Embryology of the Frog 

Sub-Phylum 11. Urochordata (Figs. 312, 313). 

Order 1. Larvacea (Appendicularia), free-swimming forms with 
permanent tail. 

Order 2. Ascidiacea (Tunicates or Sea-Squirts), fixed forms with- 
out tail in the adult. 

Order 3. Thaliacea (Salpians), free-swimming forms without tail 
in the adult. 




,...,[( f 





"■" ^^Ki -' '^"^^ 



rjrjniid % '^ffi^^ 


ocsap j^ 

f ^ph 




_A. __, 


"' i~ianr 


Fig. 313. 

I. Oikopleura in 'house'. The arrow shows course of current. 

II. Diagram of Appendicularia from the right side, an, anus; ht, heart; int, 
intestine, ne, nerve; ne', caudal portion of nerve;', principal nerve 
ganglion;".,"'., first two ganglia of tail nerve; noto., notochordj 
oes., oesophagus; or.ap., oral aperture; oto, otocyst (statocyst) ; peri-bd., peri- 
pharyngeal band; ph., pharynx; tes., testis; stig., one of the stigmata; stom., 
stomach. (After Herdman.) 

III. and IV. Ascidia. Entire animal as seen from the right side and dissection 
from the same side, an, anus; atr.ap, atrial aperture; end, endostyle; gon, gonad; 
gonad, gonoduct; hyp, neural gland; hyp.d, duct of neural gland; mant, mantle;, nerve-ganglion; oes-ap, aperture of oesophagus; or.ap, oral aperture; ph, 
pharynx; stom, stomach; tent, tenacles; test, testes. (After Herdman.) 

Sub-Phylum III. Hemichordata (Fig. 314). 

Order 1. Enteropneusta, including worm-like forms such as Balano- 

Embryology of Tadpole and Chick 


Order 2. Petrobranchiata, sessile, tube-dwelling forms — Cephalo- 

discus and Rhabdopleura. 
Order 3. Phrononidia, tube forms— Phoronis (Fig-. 199, Vol. I). 
Sub-Phylum IV. Vertebrata (Craniata). 




Fig. 313. 

V. Botryllus violaceus. 

VI. Composite Ascidian. Diagram of an individual member of a colony ot 
composite Ascidians. The zooids are in pairs and seen in vertical section, an, 
anus, at, atrium; at', atrium of adjoining zooid; cl, cloaca common to both 
zooids; end, endostyle; gld, digestive gland; gn, nerve ganglion; ht, heart; hyp, 
neural gland; lang, languets; mant, mantle; or.ap, oral aperture; ov, ovary; periph, 
peripharyngeal band; ph, pharynx; red, rectum; stom, stomach; te, testis; tent, 
tentacles; tst, test or common gelatinous mass in which individuals are imbedded; 
v.d, vas deferens (V, after Mile-Edwards; VI, after Herdman.) 

VII. Salpa democratica, asexual form, ventral view, and VIII lateral view in 
section, '^c^t, atrial cavity; atr.ap, atrial aperture; br, branchia; branch, dorsal 
lamina; ac, ciliated crests on edge of branchia; c.f, ciliated funnel; d.l, dorsal 
lip; end, endostyle; ey, eye; gl, digestive gland; gn, ganglion; ht, heart; int, 
intestine; Ing, languet; mo, mouth; mus-bds, muscular bands;, nerve- 
ganglion; CO, oesophagus; oe.ap, and or.ap, apertures of oesophagus and mouth; 
ph, pharynx; pp, peripharyngeal band; proc, processes at posterior end;, 
sensory organ; st, and stol, stolon; st, on left, stomach; v.l, ventral lip. iVII, 
after Vogt and Jung, VIII, after Delage and Herouard.) 

Order 1. Cyclostomata (round mouth eels), such as hagfish and 

Lampreys (Fig. 312). 
Order 2. Pisces (true fish with jaws). 


The Embryology of the: Frog 

Order 3. Amphibia (vertebrates with aquatic larvae, but usually 

air breathing- in the adult condition), (Fig. 315). 
Order 4. Reptilia (cold-blooded, air-breathing vertebrates). 
Order 5. Aves (birds, feathered vertebrates). 
Order 6. Mammalia (beasts or quadrupeds). 



Fig. 314. 

I. Various types of Enteropneusta which are relatives of Balanoglossus. A, 
Balanoglossus clavigerus ; B, Glandiceps hacksi; C, Schizocardium brasiliense; D, 
Dolichoglossus kowalevskii; a, anus; ab, abdominal and caudal regions; b, branchial 
region; c, collar; g, genital region; gp, gill-pore or branchial cleft; an, genital wing; 
h, hepatic region; m, position of mouth; p, proboscis; t, trunk. (From Newman, 
A, B, C, after Spengel, D, Bateson.) 

IT. A and B Rhabdopleura, C, Cephalodiscus dodecalophus. A and C, entire 
animals. B, diagram of median longitudinal section of ^. A, a, mouth;, b, anus; 
c, stalk of zooid; d, proboscis; e, intestine; /, anterior region of trunk; g, one 
of the tentacles. (After Ray Lankester.) B, a, arm; an, anal prominence; col, 
collar;, collar nerve, c.s, cardiac sac; int, intestine; m, mouth; ntc, noto- 
chord; oe, oesophagus; pr, proboscis; pr.c, proboscis-coelom; ret, rectum; st, stomach 
te, tentacles; tr.c, trunk-coelom; v.n, ventral nerve. (B, after Schepotieff, C, after 


The frog is usually considered a transitional form separating the 
lower from the higher craniata both embryologically and anatomically. 
And, although the craniates vary considerably among themselves, the 
frog has enough in common with all such variations to make it a norm, 

Embryology of Tadpole and Chick 


or standard type, for constant and valid reference, both as to anatomy 
and development. The adoption of a standard makes an understanding, 
possible of any special modifications one may find. 

Then, so much work has been done in this field that a thorough 
understanding of the embryology of the frog is essential to anyone who 
wishes to do advanced work in the zoological sciences. 

Just as there are various groups of birds with different hatching 
periods, so different species of frogs also vary as to the length of time 
the embryo requires before being able to emerge from the tgg. But, 


Fig. 315. Examples of tailed and tailless Amphibia. 

unUke the birds, the frog passes through a process of metamorphosis, 
which simply means that even after the embryo's emergence from the 
Qgg, it does not have the adult form, but must pass through still further 
changes before becoming a full-fledged frog. In the frog the form which 
is assumed at the time of hatching and which later changes on arrival 
at adult life, is called the tadpole stage. 

Temperature has much to do with both the rapidity with which a 
frog's Qgg develops, and with which the tadpole develops into an adult 
frog. This, however, is not unlike the hen's egg; for, it will be remem- 
bered that after the hen's egg was fertilized and the embryo had already 
begun to develop (before the egg has been laid), such an egg could be 
placed in a, moderately cool place for many days, which would result in 
all development ceasing. If the egg is then placed under a hen or in 
an incubator, the embryo again begins to develop. 

We shall arrange our study of the frog under two headings : First, 
the true embryonic period. This extends from the time the egg is fer- 
tilized through blastulation and gastrulation. During this time the germ 
layers as well as the larval and tadpole organs form. This period 
extends up to the time the tadpole emerges from the egg. 

Second, the larval period, from the time of hatching to the time when 
the legs ar^ formed, the tail is thrown off, and the animal has become a 


The Embryology of the Frog 

full-fledged frog with all its various organs and the form which it is to 
retain throughout adult life. 

To grasp fully that which follows, it is necessary to review the 
account of the reproductive organs in the chapter on the frog in Volume 
I of this work. 

As the sperm from the male frog never enter the female body, the 
egg must be fertilized after it has been laid. This is quite different from 
fertilization in the hen. During the breeding season, as the eggs are 
squeezed from the female, the male passes over the eggs and deposits 
his sperm upon them. The fertilized eggs begin to divide almost imme- 
diately, and within approximately thirty-six hours the blastula stage has 
been reached. In about six days, when the embryo is five millimeters in 
length, there is already a twitching within the egg, showing that life is 
present, and within two weeks after fertilization the embryo wriggles 
its way out of the surrounding jelly and becomes a free-living larva or 
tadpole. This is the end of the true embryonic period. 

__ If the temperature is 

higher than normal, such as 
it usually is in the labora- 
tory, then the larvae may 
hatch in five days. In either 
case, h o w e V e r, suckers 
(Figs. 316, 317, 318) are 
formed in the head of a tad- 
pole by which it attaches 
itself to a jelly-like substance 
surrounding the eggs, al- 
though it may attach itself 
to other objects in the water 
as well. Sometimes the tad- 
poles will even fall to the 
bottom of the water and lie there. 

As the mouth opening does not form until two to five days after 
hatching, the tadpole naturally cannot take in any food from the out- 
side, and so is still dependent upon the undigested yolk within its 

So soon as the 
tadpole begins to take 
in food from the out- 
s i d e, the suckers 
deteriorate and disap- 
pear. From this time 
on the tadpoles are 
very active. They feed 
on almost any plant 

Fig. 316. 

A frog embryo at the stage of hatching, an., 
Proctodaetim ; aii.c, slight swelling over the rudiment of 
the ear; e.g., external gills on gill arches; na., invagina- 
tion to form nasal capsule; o.c, slight swelling over the 
rudiment of eye; s., sucker; stm., stomodaeum (in- 
vagination which will form the mouth. (After Bor- 

Fig. 317. 

Four stages of the development of the adhesive apparatus 
(suckers) of Bufo vulgaris; A, suckers; M, mouth; Sp.T, 
spiracular tube. In 3 the gills are almost completely hidden by 
the united right and left opercular folds. 

The small outline figures indicate the shape and approximate 
size of the tadpoles. (After Thiele.) 

Embryology of Tadpole and Chick 125 

or animal debris, and in the laboratory will thrive on a diet of cereals. 
As the egg is dependent upon temperature for its rate of speed in 
developing, so the rate of speed at which the tadpole grows is dependent 
upon the quantity of food it obtains. 

External gills used as respiratory organs develop shortly after 
hatching. These disappear as soon as the mouth opens, and the true 
internal gills are formed. When the true gills form, they are protected 

by a cover, called the operculum. The por- 
tion underneath the operculum remains con- 
nected with the outside by only a single pore 
on the left side, known as a spiracle. The 
limb-buds appear normally at about four or 
five weeks, although in the laboratory, at a 
^ j^. ^jg * higher temperature, much sooner. The 

1, Front view of the mouth of a anterior pair develop within the opercular 

tadpole of Rana temporaria show- . -^ ^ j- 

ing the transverse rows of tiny cavity, and Consequently cannot, be seen 

horny teeth; 2, Three successive , . , ;_, . - - 

horny teeth highly magnified. (After from the OUtSlde. 1 he pOStcriOr deveiOp, 

one on each side of the cloaca a little later, 
and become quite large and jointed by the end of the second month. 

In the meantime the lungs have been growing, and the young tad- 
pole comes to the surface of the water to expel small bubbles of air and 
to take in a fresh supply. In the comm.on species of frogs, metamor- 
phosis begins at about the end of the third month. It is at this time 
that the tadpole ceases feeding, and the outer layer of skin, as well as 
the horny jaws (Fig. 318), are thrown oflf. The lips shrink, the mouth 
is no longer suctorial and becomes much wider, while the tongue 
increases in size. The eyes also become prominent. The fore-limbs 
appear, the left one pushing through the opening of the gill chamber, 
while the right pushes its way through the opercular fold on that side, 
leaving a ragged hole. The stomach and liver enlarge, while the intes- 
tine becomes shorter and smaller in diameter than before, and the 
animal becomes carnivorous. The gill-clefts close and many changes 
occur in the blood vessels due to the change in the animal's mode of 
breathing. The bladder is formed, the kidneys undergo changes, and 
there is a definite sexual differentiation. The tail shortens and is finally 
lost as the hind legs continue to lengthen. 

If, however, the water has been particularly cold, the metamor- 
phosis may be put ofif until the following spring; in fact, it seems normal 
with some species to wait even longer than this, namely, as long as two 
years, and sometimes three (Necturus) before the adult form is assumed. 

A frog's egg is somewhat akin to a chicken's egg which has been 
laid without a normal shell. The yolk is a rather blackened mass with 
a jelly-like substance surrounding it, similar to the white of the hen's 
egg, but without a solid shell. Great masses of the eggs are found in 
one place, appearing very much as though dozens of hen's eggs were 

126 The Embryology of the Frog 

broken, but with the yolks entire. The eggs vary in different species 
from one and five-tenths miUimeters in diameter to twice that size. A 
little over half of the egg is quite black, due to the pigment granules 
contained therein, while the remainder is rather white although, again, 
in different species the quantity of pigment may vary greatly. The 
darker portion is commonly known as the animal pale and the lighter 
the vegetal pole. ' 

There are three membranes covering the tgg : primary, that 
known as the vitelline membrane. This can sometimes be distinguished 
from the pigmented substance lying directly beneath it although some 
writers deny that it exists at all. 

The secondary membrane (sometimes called the chorion) is a rather 
thin but tough layer secreted from the follicle cells of the ovaries. 

The tertiary membrane is a thick jelly-like layer derived from the 
walls of the oviduct, lying close to the chorion, first as a dense layer, 
but later, as it enlarges, it becomes quite clear. 

It will be remembered that the yolk granules were quite evenly dis- 
tributed in the yolk of the hen's tgg and that the embryo developed upon 
the yolk. In the frog, however, the deutoplasm, or food part of the yolk, 
all lies at one end — the vegetal pole. Frog's eggs are, therefore, said tp 
be telolecithal. 

The nucleus lies in the animal pole and has already begun to 
divide by the time the egg is laid. In fact, it is already in the metaphase 
of the second polar division at that time. The first polar body has been 
thrown out and can be seen as a very tiny light spot in the flattened 
area of the upper pole. 

As the reproductive organs of the adult have just been reviewed 
we shall not again discuss them here ; the development of these organs 
will be taken up individually at a little later period. The general 
development of egg and sperm are quite like that which occurs in the 
germ cells of the chick. 


The sperm drills its way through the thin jelly of the chorion^and 
normally enters the egg substance in the pigmented region. The point 
of entry is a meridian passing through both poles of the egg. The 
meridian which passes through the animal and vegetal poles of the egg, 
as well as through the point where the sperm enters, is called the fertili- 
zation meridian. Only one sperm normally enters the egg. Polyspermy 
is not, however, rare, but so far as we know, always results in some 
abnormal development when it takes place. 

It will be remembered that, after one-half of the chromosome mate- 
rial of the nucleus of the egg has been thrown out by the two' polar 
divisions, the nucleus, which then contains one-half the normal number 
of chromosomes, is called the female pro-nucleus. The head of the sperm 

Embryology of Tadpole and Chick 


(which also has only half of the normal number of chromosomes) now 
enters the egg and leaves a trail of pigment behind it. This sperm, after 
entry into the tgg, becomes the male pro-nucleus. The head of the sperm 
makes its way directly to' the female pro-nucleus. The tail of the sperm 
is thrown off although both tail and midpiece enter the penetration path 
(Fig.^319). The head and midpiece, after traveling for some distance in 
the egg, rotate so that the midpiece is placed in advance of the head. 
The midpiece then begins to dissolve, and to form a typical nucleus with 
an opening within. This opening is called a vesicle. The sperm then 
changes its course and moves toward the point where male and female 
pro-nuclei will unite, unless, of course, the penetration path has already 
led in that direction. The path made by the changing of direction of the 
head of the sperm is called the copulation path (Fig. 319). This path is 
also marked by a trail of pigment as the head of the sperm passes through 
the cytoplasm to reach the female pro-nucleus. 

Sections through the egg of R. fusca^ showing penetration and copulation 
paths, and the symmetry of the first cleavage plane. A, Sagittal section through 
the egg before the appearance of the first cleavage; B. Frontal section of the 
same stage as A, showing the symmetrical distribution of the egg material. C. 
Frontal section through egg in two-cell stage, showing the symmetry of the egg; 
the penetration path is not shown, a, Anterior; cp, copulation path; /, left; p, 
posterior; pp, penetration path; r, right; s, remains of first cleavage spindle; 
sp, superficial pigment; 1, first cleavage furrow. (After O. Schultze.) 

After the sperm has entered the tgg, some of the fluid from the 
proper is withdrawn into a space between the Qgg itself and the chorion. 
This is known as a perivitelline space. The egg can thus rotate within 
its membrane. From this time onward the pigmented pole is always 
uppermost. In unfertilized eggs, the membranes are more or less 
adherent. The jelly-like covering of the egg absorbs considerable fluid 
and swells up in about a minute after the egg touches water. A close 
observation of the jelly shows that it is made up of various layers whose 
function is not only to protect the egg from chemical and mechanical 
injuries and from being eaten by other organisms, but also to elevate the 
temperature of the egg. They accomplish this latter by being trans- 
parent spheres which condense the heat rays from the sun and at the 
same time check the radiation from the egg itself. 


The Embryology of the Frog 


It has been stated that by the time the sperm enters the egg, the 
second polar division has already taken place, or rather, the metaphase 
of the second division is in the process of taking place. This division 
is completed rapidly and cuts off the second polar body in about thirty 
minutes after the sperm enters, the egg. The second polar body is either 
the same size or smaller than the first. The egg-nucleus then assumes 
its usual form. The polar bodies are often seen floating about in the 
perivitelline space. 

The male and female pro-nuclei now move toward the center of the 
egg and meet in the usual manner. The female pro-nucleus does not 
leave any pigment in its trail as does the male. The sperm centrosome 
and centrosphere divide to form the poles of a small but typical cleavage 
figure which is always located toward the animal pole ; never in the 
center of the egg. 

Immediately after fertilization there is a streaming of the formative 
protoplasm upward and the deutoplasm downward so that the animal 
pole obtains practically no yolk and the vegetal pole is composed almost 
entirely of it. At this time also, the pigment granules, directly opposite 
the point where the sperm enters the egg, are carried away leaving 
a somewhat crescent shaped lighter area. This crescentic area extends 
from half to two-thirds the distance around the egg and is known as 
the gray crescent (Fig. 320). This moving of the heavier portion to one 
side changes the specific gravity of the egg so that the portion possess- 
ing least weight lies uppermost and close to the gray crescent just 
opposite the point of entry of the sperm. 

lJv,fet<i I 1 1 xec/ 

Fe r-t i I \ -TL&dL 


Fig. 320. 

Frog's eggs showing formation of gray crescent from side and from vegetal pole. 
The animal pole is heavily pigmented. 

In about an hour and a half after the entrance of the sperm, in Rana 
fusca, according to Bracheti, the egg has arranged itself in the manner 
described, and is now ready for the first cleavage. A vertical plane is 
drawn through the point where the sperm enters the egg and passes 
over the top of thq egg through the egg-crescent. This becomes the 
midline on both sides of which the bilateral embryo is to develop. 

There are three distinct substances of varying specific gravity in 
the frog's egg, namely, protoplasm, pigment, and deutoplasm. These 

Embryology of Tadpole and Chick 


do not arrange themselves in the manner described until after fertiliza- 
tion, so that we may say that bilateral symmetry in the frog's egg is 
potential but not actual until after fertilization and the rearrangement 
of these three different substances. 

^ The original cleavage plane lies at right angles to the Qgg axis, but 
not at right angles to what is to become the axis of the embryo itself. 
There is no direct relation between the plane of the first furrow in 
cleavage and the fertilization meridian. The midline of the developing 
embryo and the penetration path of the sperm normally correspond to a 
vertical plane known as a gravitational plane, drawn through the egg 
after the particles of protoplasm and deutoplasm have rearranged them- 
selves according to gravity. All of these correspond to the first cleav- 
age furrow, though many variations of this occur. 


One of the chief reasons for studying the embryology of the chick 
before that of the frog is that the three germ layers of the chick are' 
more readily seen. The frog's egg divides into two portions, then into 
four, eight, etc., quite like the hen's egg, and by^the time there are eight 
cells present, the four cells in the region of the animal pole are found 








^ 'Kxi- 







.* " ■ 




Fig. 321. 

Cleavage of the frog's egg. A, Eight-cell stage; B, beginning 
of sixteen-cell stage; C, thirty-two-cell stage; D, forty-eight-cell 
stage (more regular than usual); E, F, G, later stages; H, I, forma- 
tion of blastopore. The central light area in / is the yolk-plug 
while the ring which encases the yolk-plug is the margin of the 
blastopore. (After Morgan.) 


The Embryology of the Frog 














-/- Vc/rr S-.c^^/e^ 





' ^ 

^~ — 


__:, — — ^■'^^^^/^^..r.- 

— K'£i"^^.' Cre^t 


^ . .„..■./... 

_^ . „_ ^ — ^c.,^ 


-^-'^-'^> . 

to be smaller than the 
fouf in the region of 
the vegetal pole. The 
smaller ones are called 
micromeres and the 
larger ones macro- 
meres. The micro- 
meres, after the fifth 
cleavage begins, divide 
more rapidly than do 
the macromeres. By a 
continually more rapid 
growth of this kind, 
the smaller animal cells 
soon almost surround 
the vegetal or yolk sub- 
stance. As the yolk 
becomes surrounded 
more and more, there 
is a somewhat central 
region where the yolk 
can still be seen from 
the exterior. This por- 
tion of the yolk is 
called the yolk-plug, 
while the margin of 
darker animal cells im- 
mediately surrounding 
the yolk-plug is called 
the blastopore (Figs. 
321, 323). 

Fig. 322. 

First two figures, photographs of Frog Blastulas. A to F, median sections through Blastulas and 
Gastrulas of A, Cynthia bipartia with 64 cells (this is a member of the tunicates). B, Gastrula ot 
Ciona intestinalis (also a tunicate). C, Gastrula of Amphioxus; D, Blastula oi Axolotl (Mexican 
salamander which breeds in the larval stage); E, Gastrula of an early stage of Turtle {Cheloma 
caouana), F, gastrula of a later reptilian stage (in the Gecko), (A to F after Babl and Van Beneden). 

Embryology of Tadpole and Chick 


The fact that the entire yolk comes to lie within the blastoderm, 
causes much of the growth process to be hidden from view. In fact, 
Professor Johnstone of Cambridge University suggests that the frog 
embryo may not have the three regular germ layers at all. We think 
they are present although pressed together so closely that it is practi- 
cally impossible to distinguish them. 

A slight separation of the darker cell layer in the yolk-plug region 
leaves an opening called the segmentation cavity or blastocoele (Figs. 
322, 323). The portion of the pigmented layer, which has separated from 
the yolk-plug, will now be known as the dorsal lip of the blastopore (Fig. 

The segmentation cavity forms near the animal pole. The entire 
blastula, however, is not much larger than the original egg because, 
although there are now thirty-two to sixty-four cells definitely formed, 
these have been formed by constant cell division without much growth 
after dividing. The roof and walls of the segmentation cavity are, there- 
fore, composed of the external pigmented animal cells. These cells are 
of different shapes and sizes, rather irregular and loosely arranged, and 
really divided into two sheets, one lining the blastocoele while the other 
forms the true outer layer of the blastula. It will thus be seen that 
the frog blastula is not made up of a single layer of cells but of a double 

Fig. 323. 

Median sagittal sections through a series of gjastrulas of the frog (R. tern- 
poraria). The figures illustrate the change in position of the whole gastrula. as 
well as the phenomena of gastrulation proper. A. Commencement of gastrulation; 
earliest appearance of the dorsal lip of the blastopore. Internally the gastrular 
cleavage is indicated. B. Invagination more pronounced; beginning of epiboly. 
C. Invagination, epiboly and involution in progress. The gastrular cleavage is 
now indicated on the side opposite the blastopore. Rotation of the gastrula. D. 
Just before the ventral lip of the blastopore reaches the median line. The in- 
dentation of the wall .of the segmentation cavity is an artifact. E. Blastopore 
circular and filled with yolk plug. Gastrula beginning to rotate back to its original 
position. Peristomial mesoderm differentiating. F. Segmentation cavity nearly 
obliterated. Neural plate established. G. Gastrulation completed, a, Archenteron; 
h, blastopore; c, rudiment of notochord; ec, ectoderm; en, endoderm; gc, gastrular 
cleavage; ge, gut endoderm; m, peristomial mesoderm; np, neural plate; nt, transverse 
neural ridge; s, segmentation cavity or blastocoele. (After Brachet.) 

132 The Embryology of the Frog 

layer of animal cells. This double layer is the ectoderm. The floor of 
the blastocoele is made up of the large vegetal cells. 

As the pigmented animal micromeres divide more rapidly from now 
on, they naturally must grow toward the equator. This causes a thin- 
ning of the roof but a thickening of the walls of the segmentation cavity. 
The equator of the blastula seems to be the region in which the cells 
multiply most rapidly, and this equatorial region is called the germ-ring 
or growth zone (Fig. 324). 

At the time when the germ-ring begins its rapid multiplication of 
cells, the gray crescent extends downward rather rapidly. This region 
is to become the posterior, or caudal, side of the embryo. The germ-ring 
from now on continues extending beyond the equator into the vegetal 
region until it lies approximately half way between the equator and veg- 
etal pole. This growing of the germ-ring pushes the yolk more and 
more within the overgrowing animal cells as already mentioned. The 
yolk thus being pushed within, naturally forces the floor of the segmenta- 
tion cavity into a convex arch. 

This is considered the end of blastulation in the frog's egg. 


We have at this point, then: most of the yolk withdrawn within 
the overgrowing animal cells, two layers of which form the outer cover- 
ing of the blastula at the animal pole; a segmentation cavity with its 
floor convexly arched ; and a definite antero-posterior differentiation, the 
posterior side being marked by the gray crescent. 

The development of the gastrula begins just beneath the posterior 
lip of the blastopore by a groove which forms directly between the ani- 
mal cells and the yolk cells (Fig. 323, A, gc). The groove itself is lined 
by both kinds of cells on its opposite faces. We know from the develop- 
ment which takes place later that this groove is the real beginning of 
invagination. The groove itself becomes the archenteron or primitive 
intestinal tract (Fig. 323, C, E, F, G, a). The upper lip of this groove is 
the rim of the blastopore (Fig. 323, b). The animal cells become the 
ectoderm and the yolk cells become entoderm. 

As the yolk is pushed within the blastula it causes a narrow groove 
to form in the region of the blastocoele. This groove separates the rising 
floor from the remaining yolk and finally becomes a definite narrow 
slit which splits off the ectoderm and entoderm at the point of invagina- 
tion. The original groove is known as the gastrular groove, and the 
splitting off into ectoderm and entoderm is called gastrular cleavage 
(Fig. 323, A, gc). This gastrular cleavage extends from the dorsal lip 
of the blastopore entirely around the gastrula to the opposite side from 
where invagination takes place. 

In the invagination area a definite tongue of ectodermal cells pushes 
inward (Fig. 323, C, en) to join directly with the inner yolk cells to 

Embryology of Tadpole and Chick 133 

form the entoderm. Due to their position, the inner yolk cells are also 
entoderm, although they do not form by a true invagination. 

Viewing the entire egg externally, during the process of gastrula- 
tion, we may consider the germ-ring as something like a rubber band 
placed about an ordinary ball in an equatorial plane. By sliding the 
rubber band off the ball toward one side, we may understand how the 
germ-ring brings its lateral region together in the mid-rim in the pos- 
terior or caudal reg-ion. This coming together not only pushes the 
underlying cells within, but causes the entoderm to extend further inward 
and thus increases the cortical extent of the archenteron. 

We, therefore, have the first invagination of the pigmented cells 
forming the dorsal lip of the blastopore ; the invagination then extends 
laterally in both directions to form the lateral lips of the blastopore; and 
finally the process of invagination continues around to the side of the 
gastrula, practically to a point almost opposite to where it began, and 
where the ventral lip of the blastopore is formed. This completes the 
entire blastopore. After the yolk plug has disappeared within the gas- 
trula, the blastopore remains a narrow, elongated slit connected with 
the archenteron. 

The method by which the blastopore grows by concrescence to 
form the primitive streak has already been described in the case of the 
chick, a rereading of which should be done at this point. Comparisons 
should constantly be made between the development of frog and chick 

It will be noted from what has been said that gastrulation is not so 
much formed by invagination in the frog as it is by a delamination within 
the gastrula itself. 

In fact, among the higher chordates, invagination is sometimes 
entirely lacking, so that gastrulation may be accompanied by either 
involution, such as takes place in the chick, or by epiboly, which occurs to 
some extent in the frog, and by delamination, a process just described. 

At this point Figures 321, 323 must be studied to understand the 
varying changes of positions of the blastopore brought about by the 
rotation of the egg. It is also to be remembered that the blastopore 
marks the caudal extremity of the embryo. 

The two-layered stage in the frog is of very short duration. 

In the inner region, where the germ-ring and the yolk-cells which 
line the blastocoele are continuous, there are transitional cells which are 
to become the mesoderm (Fig. 324, m). These cells are continuous with 
ectoderm on one side, and entoderm or yolk-cells on the other, and can 
not be distinguished as definite mesodermal cells until the blastopore is 
completely formed. 

In other words, the mesoderm first appears as a ring; of cells just 
within the margin of the blastopore. This mesodermal region is broad- 
ened considerably in the dorsal region. 


The Embryology of the Frog 

As the blastopore closes, it carries mesodermal cells toward the mid- 
line (Fig. 324) to form a broad median band extending forward from 
the dorsal lip of the blastopore. These cells then multiply and, as the 
dorsal lip extends downward, an axial thickening is formed. At this 
same time the archenteron extends and carries yolk-cells outward toward 
the animal pole so that the extent of the mesoderm is almost as great as 
that of the entoderm. 

Fig. 324. 

Frontal and transverse sections through gastrulas of the frog (J?, temporaria) 
of various ages. A. Frontal section through gastrula of same age as Fig. 323. 
C. B. Frontal section through gastrula of same age as Fig. Z22>, D. C. Frontal 
section through gastrula slightly older than Fig. 323, F. D. Frontal section 
through gastrula of same age as Fig. 32, G. E. Transverse section through gastrula 
slightly older than Fig. 323, D. F. Transverse section through gastrula slightly 
older than Fig. 323, G. a, Archenteron; b, blastopore; c, notochord; ge, gut 
endoderm; m, peristomial mesoderm; np, neural plate; s, segmentation cavity or 
blastoccele. (After Brachet.) 

Then a rearrangement of cells takes place so that an irregular 
delamination begins in the dorsal lateral regions on each side of the 
thickened axial mass. This delamination extends from there anteriorly 
and laterally around the sides of the archenteron. Thus a thick layer of 
mesoderm is formed between the entodermal lining of the archenteron 
and the outer ectoderm (Fig. 324, m). 

At the lower pole, that is, toward the place where the yolk-plug is 
being drawn into the interior of the Qgg (Fig. 324, b), the lower surface 
of the yolk is also delaminated so that a circular margin of the mesoderm 
is formed there. It is from this layer of mesoderm that cells and groups 
of cells bud off and pass toward the lower pole — (it is to be remembered 
that these cells and groups of cells begin their growth in the lower pole 

Embryology of Tadpole and Chick 


region, but lie above the lower pole itself) — toward the ventral portion 
of the blastopore, so that a more or less completely continuous layer of 
mesoderm is formed between the ectoderm and entoderm. 

In the dorsal region of the blastopore, and extending along the dor- 
sabaxial mass, delamination does not occur as rapidly as in the ventral 
region. The course of delamination is also modified here. This modi- 
fication is no doubt due to the fact that in this dorsal axial region the 
cells, which are to become mesoderm, are derived from the cells which 
invaginated from the outer layer, while in other regions this is not the 
case. Then, too, it is in this region that the notochord forms, which 
further complicates matters. 

In fact, cross sections in the region of the blastopore do not show 


f - • • -f il;^ ^ . 7>f,^^y^y 

^^^'^X w^ 

■ Ofn/ Cuctrer 

<f/'/a£. tdty '^/Y. 


4/.'c /i^sercA 



Fig. 325. 

A, B, C, Transverse sections of Amhly stoma tadpole. A, through suckers; B, through optic and 
olfactory region; C, through gill region. 


The Embryology of the Frog 

lines of demarcation between the notochord, mesoderm, and the dor- 
sal entoderm for some little time; but sections through the blastopore 
(while the yolk-plug is still protruding) show the rim of the blastopore 
to be composed of thick, undifferentiated cells which are a part of the 
contracted germ-ring. A little laterally, the ectoderm is separated by 
a narrow space or line which has formed during gastrulation, and the 



Fig. 325. 

D, E, F, Photographs of transverse sections through 12 mm. frog tadpole. 
The sections are cut diagonally; the left eye (right side of figure) lying more 
cephalad, is the most anterior. E, immediately posterior to D, but lens was lost in 
sectioning; F, immediately posterior to middle cut. 

Embryology of Tadpole and Chick 


thin entoderm is separated from the middle layer. This separation of 
layers is the delamination process we have been discussing. 

As the outer animal cells are pigmented, we assume that whenever 
pigment is found in any of the inner cells, it is an indication that such 
pigmented cells are derived from the ectoderm. 

As the yolk-plug is withdrawn, the blastopore becomes a mere slit, 
and the ectoderm and mesoderm separate considerably and dip down- 
ward toward the entoderm in the dorsal midline although never so far 

Fig. 325. 

138 The Embryology of the Frog 

as to reach the entoderm. This leaves a narrow, vertical ridge of cells 
at the midline. 

A pair of slight depressions from the archenteron appear only as 
virtual grooves formed by pigmented cells. Forward from this, the 
lower margins of these grooves come to look like lips which approach the 
midline. The mesoderm in the meantime separates from the archenteron 
to a considerable extent (Fig. 324). 

Still farther forward, the grooves disappear, while the spaces 
between mesoderm and entoderm, as well as between mesoderm and ecto- 
derm, extend vertically and delimit the pair of mesoderm masses. 

The cells thus left in the midline between the mesodermal sheets 
formx a wedge-shaped elevation which is continuous with the entoderm. 
This is the beginning of the notochord. (Figs. 323, G, c, and 324, F, c). 

The notochord is now cut off from the entoderm by a narrow slit 
which leaves the archenteron with a dorsal roof, one cell in thickness. 
(Fig. 324.) 

From this point, posteriorly (toward the blastopore), the grooves 
of the archenteron are better marked. 


At the same time that the notochord is developing, the medullary 
plate is also being formed (Figs. 324, np, and 326, everything above 
mes.), partly from the medial band of cells which extends from the 
region of the dorsal lip of the blastopore nearly to the animal pole, and 
partly from the axial thickening caused by the confluence of the lateral 
portions of the germ-ring. 

The inner portion of the former region is called the nervous layer 
of ectoderm. It is this nervous layer which begins to thicken. By 
the time the blastopore has begun to close, a thickened medullary plate 
has formed over the entire dorsal surface of the gastrula, essentially like 
that described in the chick. 

At about the time the yolk-plug has disappeared, the lateral ridges 
have become elevated to form the lateral neural ridges (Fig. 327, n, f). 

Fig. 326, 

Transverse section through frog tadpole in the anterior 
region of the neural groove, au, eye-pits. (Note the pigment 
in the outer cells); ect, outer layer of ectoderm; ent, entoderm; 
mes, mesoderm; nr, anlage of the neural crest. (After 

Embryology of Tadpole and Chick 


These extend from the blastopore to almost directly opposite on the 
dorso-lateral portion of the embryo. Here they turn sharply to pass 
toward the midline where they meet to form the transverse neural fold. 
It is this latter fold which marks the anterior limit of the medullar}^ 
plate. The median groove becomes more pronounced and is then called 
the neural groove. 

The important points to bear in mind here are: 

First, that, as in the chick and all chordate animals, so too in the 
frog, the closing of the blastopore by confluence (extending in an antero- 
posterior direction, on the dorsal aspect of the embryo) forms a definite 

— V 




an -,.^ 






Fig. Z21. 

Frog embryos. A, from behind and above; B, from in front; C, slightly earlier 
than A and B. an., proctodeum (the invagination from which anus will form) ; 
hip., blastopore; ga, gill arches; gp., gill plate; nj., neural fold; s, sucker; sp., 
sense plate. (After Borradaile.) 

axis, which means that by this confluence, the germ-ring becomes the 
axis. It is on each side of this axis that organs and important structures 

Second, that gastrulation involves only the forming of two layers 
(ectoderm and entoderm) from the single layered blastula. 

Third, that notogenesis includes all the processes involved in the 
formation of the medullary plate, notochord, and mesoderm. 

Gastrulation is accomplished in the frog chiefly by a delamination 
and rearrangement of the yolk cells and only to a slight extent by 
invagination. The process of invagination is chiefly concerned in forming 
the beginning of the notochord and the mesoderm in the dorsal and 
dorso-lateral regions. These latter structures are not formed entirely, 
however, by invagination, but also by material from the germ-ring which 

140 The Embryology of the Frog 

has been carried to the axial region. Invagination, in the frog, is there- 
fore considered of minor importance. 

Amphioxus is the only chordate which remains in a two-layered 
state for some time. All other chordates retain this condition for a short 
period; this is so because the mesoderm begins its development almost 
at the moment of gastrulation. 

In fact, in the frog, mesodermal cells are found almost immediately 
after the entoderm cells begin to form. They are found, first, all around 
the margin of the blastopore where they form an important part of the 
germ-ring. Mesoderm which forms in this way is called blastoporal or 

As confluence begins, the lateral portions of the germ-ring are 
brought closer to the mid-dorsal region where they become a part of 
the axis of the elongating embryo. That is, they form the mesodermal 
bands, already referred to, and it is these bands of the germ-ring which 
become axial in position, then to be known as gastral mesoderm. This 
gastral mesoderm is nothing more nor less than mesoderm derived in 
turn from the blastoporal mesoderm. It is no different from any other 
mesoderm derived from the same origin, although it lies, of course, in 
a different position from the remaining mesoderm. Regarding Amphi- 
oxus, however, the above statement would not be true; for, in that 
animal, gastral mesoderm and blastoporal mesoderm have different 
origins. This difference in origins may be traced to the fact that in the 
frog the mesoderm differentiates before gastrulation and confluence, 
while in Amphioxus, the mesoderm does not differentiate until after 
these two processes have begun. 


All that has been discussed so far has taken place within two days 
after fertilization. Now the embryo can be seen either lying in a 
straight line, or slightly concave, on the dorsal surface. The ventral 
surface appears convex. The ectoderm still forms the entire covering 
epithelium, although some of these ectodermal cells have developed cilia 
which appear just before the fusion of the neural folds. These cilia 
beat in a posterior direction to give the embryo a slow rotary motion 
within the egg membranes. 

The transverse neural fold marks the anterior limit of the nervous 
system, while the posterior limit is located just anterior to the dorsal 
lip of the blastopore. 

As confluence continues, the nervous system comes to extend from 
almost pole to pole on the posterior surface of the gastrula, even before 
the blastopore has entirely contracted. The gastrula then rotates so 
that this posterior portion becomes dorsal ; this brings the transverse 
neural fold to the anterior end of the nervous system, while the medul- 
lary plate occupies nearly the entire dorsal surface of the embryo. 

Embryology of Tadpole and Chick 


The ridges which form on the neural plate, and later fuse in a similar 
manner to that described in the chick, begin fusion in the region of the 
medulla (Fig. 327) and continue both anteriorly and posteriorly. The 
transverse fold extends backward to form the roof of the expanded 
brain end of the developing nervous system. It meets the lateral folds 
ius the region lying between the fore and mid-brain and as it is the 
last region of the neural tube to close, it is called the neuropore. The 
neuropore lies just posterior to where the epiphysis is to appear. It is 
quite transitory. 

Neural crests are formed in the way they w^ere formed in the chick 

The blastopore has become a narrow slit, the lateral walls of which 
fuse, leaving two openings, an upper and a lower. The upper one is 
directly continuous with the archenteron while the lower one is known 
as the proctodaeum. The proctodaeum is but a slight depression lined 
with ectoderm. 

The posterior ends of the neural folds extend out from the middle 
regions where they fuse, to form the neural tube which then covers the 
upper blastoporal opening, thus forming a connection between arch- 

Sections of an embryo frog. A, transverse; B, longitudinal, an., 
Anus; cocL, coelom; ect., ectoderm or epiblast; end., endoderm or hypoblast;, fore-brain;, hind-brain; ht., rudiment of heart; int., intestine; 
I. p., lateral plate of mesoblast; Ir, rudiment of liver;, mid-brain; 
m.s., mesoblastic somite; mes., mesoblast; mes'., mesoblast continuous with 
epiblast of neural canal and hypoblast of notochord; ne.c, neurenteric 
canal; nch., notochord; ph., pharynx; pit., rudiment of pituitary, body; 
so.m., somatic mesoblast; sp.c, ' spinal cord; sp.m., splanchnic mesoblast; 
stm., stomodseum, (After Borradaile.) 

142 ' The Embryology of the Frog 

enteron and neural tube. This opening is called the neurenteric canal. 
(Fig. 328.) 

Just as with the chick, the confluence of the two lateral walls of 
the blastopore (which have been formed from the remains of the germ- 
ring) bring a median cell mass together in the axial region, in which 
ectoderm, entoderm, and mesoderm are fused in a quite undifferentiated 
mass. This is the primitive streak. The groove which lies in the mid- 
line of the primitive streak is called the primitive groove. 

It is from this primitive streak that ectodermal cells are budded 
forth into the neural folds and upon the surface of the body. It is from 
the primitive streak, also, that mesodermal cells are budded off into the 
lateral bands, and entodermal cells into the walls of the archenteron. 

At this time the chief characteristic of the brain is the single flexure 
around the tip of the notochord (Fig. 329). The hypophysis (pituitary 
body) can be seen as a tongue of ectodermal cells just beneath the end 
of the fore-brain ; it extends inward a short distance. 

T'he rudiments of the eyes (Fig. 326, au) are indicated as small 
patches of the deeply pigmented ectodermal epithelium in an antero- 
lateral region of the medullary plate. 

The rudiments of the ears (Fig. 282, C) are seen as thickened 
patches of the inner or nervous layer of ectoderm opposite the region 
of the hind-brain. They are difficult to see externally as yet. 

The rudiments of the olfactory organs are formed as thickened 
patches of ectoderm below and in front of the optic rudiments. The tiny 
depressions on the surface which are to form the future olfactory pits 
may sometimes be seen at this period. 

The notochord is completely delaminated, except in the region of 
the primitive streak, by the time the neural tube has closed. 

By the time the neural tube is completed, the archenteron is called 
a mesenteron; the anterior enlarged end forms the fore-gut, the walls 
of which are but one cell in thickness (Fig. 329). The fore-gut region 
is also called the pharynx in the embryo. The stomach and oesophagus 
are later to be developed from this region. 

Sagittal section of Anterior end of a frog tadpole 3.6 mm. long 
(Redrawn from Corning.) 

Embryology of Tadpole and Chick 


Just in front of the neurenteric canal there is also an enlargement 
which forms the hind-gut or rectal portion of the intestine. 

The mid-gut is, as in the chick embryo, that small portion in direct 
connection with the yolk. 

It will be remembered that the true mouth forms in chordates ab 
a 'secondary inpocketing of ectoderm. In the frog, the outpocketing 
from the fore-gut, which is to meet the ectodermal inpocketing, is seen 
just below the fore-brain (Fig. 329, pharyngeal membrane). This is the 
region where the mouth will form later. 

The liver will be seen as a ventral outgrowth beneath the anterior 
end of the yolk mass (Fig. 328). 

In sections, the rudiments of the first two or three visceral pouches 
can be seen as vertical outgrowths from the sides of the pharyngeal 
walls (Figs. 295 and 330). The pouches extend to the ectoderm with 

Fig. 330. 

A. Horizontal section through an embryo frog some time before hatch- 
ing, showing the optic vesicles springing from the sides of the fore-brain, the 
three anterior pairs of gill-slits, and five pairs of mesoblastic somites. B. A 
similar section through a tadpole shortly after hatching. The head is cut in 
a lower plane than in A, so only a small part of the anterior end of the brain 
appears in the section. oS the mandibular arch; a", the hyoid arch; a^, the 
first branchial arch; bv, blood-vessel in first and second branchial arch; eg, 
external gills; ent, enteron; fbr, fore-brain; mch, branching mesenchyme 
cells; na, nasal pits; nch, notochord; nps, peritoneal funnel; op. optic vesicle; 
ph, pharynx; pnp, pronephros; som, mesoblastic somites which in B are 
converted into muscle. I, mandibulo-hyoid slit; // hyo-branchial slit; III-V, 
branchial slits. (After Bourne.) 


The Embryology of the Frog 

which they fuse. It is this fusion which causes the depressions in the 
ectoderm just back of the head on the external surface. 


Just as with the chick, somites are formed by transverse division 
along the dorsal portion of the embryo, except in the primitive streak 






■% 7lt. 


Fig. 331. 

A. Tadpole of the frog at the time of hatching, an, anus; e.r.g, external 
gills; na, nasal pit; s, sucker; som, somites; st, stomodaeum; yk, yolk-sac. 

B. Diagram of a frontal section of a frog larva at the time of hatching 
(modified), c, Coelom; d, pronephric duct; F, fore-brain; i, infundibulum; 
in, intestine; n, nephrostome; o, base of optic stalk; ol, olfactory pit (placode); 
p, pharynx; t, pronephric tubules; II, hyoid arch; III-VI, first to fourth branchial 
arches; 1, hyomandibular pouch; 2-6, first to fifth branchial pouches. 

C. "A diagram of a transverse section of the frog embrj'o at the hatching 
stage. C(£l., CcElom; ect., ectoderm; int., intestine; Ir., liver; m.pL, muscle 
plate; nch., notochord; nst., nephrostome; s.d., segmental duct; sp.c, spinal 
cord. The glomeruli are seen opposite the nephrostomes. {A and C, after 
Borradaile; B, after Marshall.) 

Embryology of Tadpole and Chick 


region and in the head region. A complete sheet of mesoderm now sep- 
arates ectoderm and entoderm in the embryo from the head region to the 
primitive streak region, and this sheet separates into two layers, an outer 
or somatopleure, and an inner known as the splanchnopleure. (Fig. 


The thickened region, where the somites form on the dorsal surface 

of the embryo along the notochord, is called the segmental plate or 

myotomal region, while the portion extending laterally (which is much 

thinner) forms the lateral plates. 

The coelom develops between somatopleure and splanchnopleure. 

As the frog's egg has the yolk- 
mass packed within the embryo, this 
mass pushes the germ layers close 
together, so that they are by no 
means as clearly set apart as in the 
chick embryO'. 

Between the second, third, and 
fourth somites and the lateral plates, 
small masses of cells remain closely 
related to the somatopleure of the 
lateral plate. It is these cell masses 
which are to develop into the pro- 
nephric tubules. All of these struc- 
tures must be compared with similar 
developmental structures in the 
chick embryo, at this point. 

The coelom proper can be seen 
as a definite space only below the 
pharynx in front of the liver (Fig. 
331). The heart will develop in the 
region where the loosely scattered 
cells are seen, ventral to the 


There are details in which the 
various species of frogs vary, but all 
pass through the following general 
method of development. 

It is both interesting and profit- 
able to call attention at this point 
to the fact that, while the frog was 
one of the earliest forms of animal 
life studied in the laboratory, and 


Fig. 332. 

Diagrams of median sagittal sections of 
the anterior ends of frog larvae. A. Of a 
larva just before the opening of the mouth. 
B. Of a 12 mm. larva (at the appearance of 
the hind-limb buds). a, Auricle; ao, dorsal 
aorta; b, gall bladder; bh, basihyal cartilage; 
ch, cavity of cerebral hemisphere (lateral 
ventricle) ; e, epithelial plug closing the 
oesophagus; ep, epiphysis; g, glottis; h, 
hypophysis; H, hind-brain; hr, cerebral 
hemisphere; ht, horny "teeth"; hv, hepatic 
vein; i, intestine; if, infundibulum; /, lower 
jaw; I, liver; ly, laryngeal chamber; m, 
mouth; M, mid-brain; mb, oral membrane 
(oral septum) ; n, notochord ; o, median por- 
tion of opercular cavity; ce, oesophagus; p, 
pharynx; pb, pineal body; pc, pericardial 
cavity; pd, pronephric (mesonephric) duct; 
pt, pituitary body; pv, pulmonary vein, pIII, 
choroid plexus of third ventricle; pIV, choroid 
plexus of fourth ventricle; r, rostral cartilage; 
ro, optic recess; s, stomodaeum; sv, sinus 
venosus; t, thyroid body; ta, truncus arteriosus; 
tp, tuberculum posterius; v, ventricle; vc, in- 
ferior (posterior) vena cava. (After Marshall.) 

146 The Embryology of the Frog 

while many hundreds of volumes and articles have been written on it 
from many angles, there are nevertheless hundreds of interesting points 
in that animal's development which are still unknown. In fact, one 
writer says that the gaps that confront one in the study of the frog 
assume "remarkable proportions" when one thinks of how much work 
has really been done on this much-studied animal. 

Then, too, as there is no accurate method of obtaining the age of 
a frog, owing to the remarkable influence temperature and food play in 
its development, it is often difficult to make clear much that should be 
made clear to the student. 

Roughly speaking, at the time of hatching, namely about one or 
two weeks after fertilization, the larvae of most species are about six 
or seven millimeters in length. The tadpole is usually about nine or ten 
millimeters in length at the time of the opening of the mouth, and eleven 
or twelve millimeters when the limb-buds appear. 


There are no neuromeres in the brain region of the frog, though 
otherwise the same brain divisions take place, which we have discussed 
in the chick. 

The tuberculum posterius (Fig. 332, tp) is a thickening opposite the 
tip of the notochord in the floor of the brain, while a dorsal thickening 
appears in the roof of the brain obliquely upward and forward from this. 
A plane passing from the tuberculum posterious in front of the dorsal 
thickening separates the fore-brain from the mid-brain, while a plane 
passing from the same tuberculum behind the dorsal thickening sepa- 
rates the mid-brain from the hind-brain. 

The beginning of the brain divisions is quite similar to the three 
primary regions mentioned in the chick embryo, and a review of the 
matter there given will make the divisions and cavities in the frog brain 

It will be remembered that the olfactory lobes and the cerebral hem- 
ispheres form the telencephalon, and that the telencephalon and the 
*'tween-brain" (diencephalon) together form the fore-brain, while the 
optic lobes and optic chiasma form the main portions of the mesencepha- 
lon. The cerebellum takes up most of the metencephalon, while the 
medulla oblongata forms the myelencephalon. 


Opposite the neuropore (Fig. 328) the cells of the ectodermal cone 
scatter, due to the pushing out of the head tissues in advance of the 
brain. Soon all trace of the neuropore disappears except for a slight 
indentation known as the olfactory recess, and this also disappears a 
short time later. 

The lamina terminalis (Fig. 282, C) is a thickening just below the 

Embryology of Tadpole and Chick 



level of the olfactory recess in the anterior wall of the fore-brain. ^ It 
extends ventrall;>^ to where the optic stalks protrude. From the drawing 
(Fig. 333) it will be noticed that a thickening occurs in the region of the 
attachment of the optic stalks to form the torus transversus (Fig. 333, tr) 
and the beginnings of the optic chiasma (Fig. 333, cw) and thalami. 
The torus transversus becomes the seat of the anterior commissure 
(Fig. 333, cpa) as well as other commissures of the brain. 

The narrow depression between the thickenings just mentioned 
forms the recessus opticus (Fig. 333, ro), which is the passage to the 
cavities in the optic stalks. 

The infundibulum (Fig. 333, J) is an outgrowth of the posterior 

portion of the fore- 
brain where it extends 
backward under the tip 
of the notochord. 

The epiphysis, or 
pineal body (Fig. 
333, e), is an evagina- 
tion from the dorsal 
wall of the fore-brain 
at its posterior limit 
where the wall has 
become quite thin. 

The choroid plexus 
(Fig. 333, pch) is the 
non-nervous portion of 
the roof of the fore- 
brain which has be- 
come thinned out 
considerably, and in 
which the blood vessels 
lie. This whole region 
is pushed into the 
third ventricle. 

The habenular 
ganglia and the haben- 
ular commissure (Fig. 
333, ch) develop be- 
tween the choroid 
plexus and the 

The paraphysis (a 

dorsal growth) devel- 

and commissure considerably 

Fig. 333. 
Median sagittal sections through the brain of the frog. A. 
Of a larva of R. fusca of 7 mm. in which the mouth was open. 
B. R. esculenta at the end of metamorphosis, c. Cerebellum; ca, 
anterior commissure; cd, notochord; ch, habenular commissure; 
cp, posterior commissure; cpa, anterior pallial commissure; cq, 
posterior corpus quadrigeminum; ct, tubercular commissure; cw, 
optic chiasma; d, diencephalon; dt, tract of IV cranial nerve; 
e, epiphysis; hm, cerebral hemisphere; hy, hypophysis; /, in- 
fundibulum; M, mesencephalon; Ml, myelencephalon Mt, 
metencephalon; p, antero-dorsal extension of diencephalon; pch, 
choroid plexus of third ventricle; R, rhombencephalon; rm, 
recessus mammillaris; ro, optic recess; se, roof of diencephalon; 
si, sulcus intraencephalicus (the groove which forms the hindermost 
boundary of the mid-brain); t, telencephalon; tp, tuberculum 
posterius; tr, torus transversus (telencephali) ; vc, valvula cere- 
belli; vi, ventriculus impar (telencephali) (third ventricle). (From 
Von Kupffer, Hertwig's Handbuch, etc.) 

Ops in front of the habenular g-anglia 

148 The Embryology of the Frog 

The cerebral hemispheres appear when the tadpole is about seven 
millimeters in length, that is, when it is ready to hatch. 

The ventricles are similar to those in the chick. 

As all these thickenings and outgrowths appear, the brain itself 
seems to straighten the original flexure, but this is only apparent, as the 
flexure remains, and the infundibulum still extends below and in front 
of the tip of the notochord. 

The hypophysis (Fig. 333, hy) grows as an inward extension of the 
surface ectoderm to meet with the infundibulum. 


There is not much change in this region of the brain except that 
the ventro-lateral walls thicken, and these thickened portions are known 
as the crura cerebri. They connect with the wall of the fore-brain. The 
dorso-lateral walls of the mid-brain form the large, rounded optic lobes. 

The posterior commissure (Fig. 333, cp) forms the anterior limit of 
the mid-brain. 

The aqueduct of Sylvius (Fig. 303) is the cavity in the mid-brain 
connecting the third ventricle with the cavity in the rhombencephalon. 


There is little, if any, line of demarcation in the frog which divides 
the hind-brain into metencephalon and myelencephalon. 

The cerebellum (Fig. 333, c) is in the region which is commonly 
designated as the metencephalon. This organ is quite small in the frog, 
and appears late in larval life on the dorsal side of the hind-brain. 

The non-nervous thinned-out roof of the fourth ventricle (which 
covers the dorsal part of the region of the medulla oblongata or myelen- 
cephalon),- (Fig. 333, Ml) forms the choroid plexus of the fourth 

The floor of the ventro-lateral walls of the hind-brain becomes thick- 
ened and forms the main nervous pathways to and from the nuclei of 
origin of most of the cranial nerves. 

The brain gradually tapers into the spinal cord at the medulla 

The central canal is the central opening running- throughout the 
length of the spinal cord. It is continuous with the cavities of the brain. 
This central canal is lined with non-nervous cells known as ependymal 
cells. The true nerve-cells, which go to make up the main portions of 
the wall of the spinal cord, are called germinal cells. These latter are 
in turn divided into supporting cells, or glia cells, and the true functional 
nerve cells or neuroblasts. 

There is no dorsal fissure in the frog's spinal cord as there is in 
higher forms, though there is a ventral fissure. 

The gray matter of the cord is formed by the neuroblasts. 

Embryology of Tadpole and Chick 



The peripheral nervous system has been discussed in some detail in 
the chick, and will be taken up in still greater detail in our study of 
comparative anatomy. 

In the tadpole there are some forty pairs of spinal nerves, but only 
ten pairs in the adult. They arise by a dorsal and ventral root, which 
unite to form the trunk of the spinal nerve, after which this trunk divides 
into a dorsal and a ventral ramus, while a ramus communicans connects 
the trunk with the sympathetic system. 

There are also ten cranial nerves instead of twelve as in the higher 

Fig. 334. 

Transverse and frontal sections o£ frog embryo to show position and division of neural 

crest in head region. 

The V, VII, IX, and X are called branchiomeric nerves on account 
of their close relationship to the branchial clefts. 

The cranial nerves take their substance from three embryological 
elements, namely : (1) the cell masses derived from the neural crests 
as described in the study of the chick, (2) cells from ectodermal patches 
on the surface of the head, and (3) from the cell processes which extend 
outward from the neuroblasts in the ventro-lateral walls of the spinal 

They differ, therefore, from the spinal nerves, for in these (2) is 

The V, VII, IX and X cranial nerves arise by a single root (though 
this may be mixed, i. e., it may be both sensory and motor in function) 
to pass into a large ganglion. Beyond this ganglion a large horizontal 
branch is given ofif which in turn branches into two rami which pass 
anteriorly and posteriorly to the gill cleft with which the particular 
nerve is associated. 

As in the chick, so in the frog, the cranial nerves develop from the 
neural crests left on each side of the central canal after the neural folds 

150 The Embryology of the Frog 

fuse and the indented ectoderm again returns to its normal condition. 
The neural crests are thus left between the central canal and the outer 

The neural crests are quite large in the head region, becoming 
smaller toward the posterior region of the embryo. Each crest becomes 
divided into three masses as the neural plate begins to close. (Fig. 334.) 

The more anterior of these divisions, which lies in the region of the 
mid-brain, is the beginning of the V nerve; it forms the trigeminal 
ganglion. The middle section is the beginning of the VII and VIII 


Fig. 335. 
The Nerve Placodes in the head of an Ammocoetes 4 mm. long. V^, first 
ganglion of the V cranial nerve; V'^, second ganglion of the same nerve ;_ VII, 
ganglion of the VII cranial nerve; IX, ganglion of the IX nerve; X, ganglion of 
the X (Vagus) nerve, 1, 8, 13, first, eighth and thirteenth ganglia in the epi 
branchial series, br^, hr^, first and eighth branchial pouches; c, ciliary nerve; ch, 
notochord; /, lateral ramus of the X cranial nerve; N, anlage of hypophysis; n.a.,. 
VI cranial nerve (abducens) ; o., ophthalmic nerve; y./., recurrent branch of VII 
cranial nerve (facial); r.v., recurrent branch of X (vagus) nerve; t., TV cranial 
(trochlear) nerve; ves.op., optic vesicle. (From Vialleton after von Kupffer.) 

nerves ; it is known as the acustico-facialis ganglion ; while the posterior 
division forms the beginning of the IX and X nerves, or the glosso- 
pharyngeal and vagus ganglia. The three divisions do not separate 
entirely from the medullary plate, but remain connected by a very 
slender chain of cells to the medullar region of the brain. 

When the tadpole has developed three or four somites, the inner or 
nervous layer of the ectoderm opposite the crest ganglia proliferates to 
form a patch, sometimes three or four cells m thickness. Such patches 
are known as placodes (Figs. 268, 335), and are thought to be vestigial 
sense organs. 

In the placode there is found a superficial sensory element, which 
may disappear, and a deep ganglionic element which is usually retained. 
It is the ganglionic portion which fuses with the nearest crest-ganglion 
to form the principal sensory portions of the nerve. 


This is the principal nerve of the mouth and mandibular arch. The 
trigeminal portion of the neural crest is large and extends from the eye 
to the hyomandibular cleft (Fig. 335). The ectodermal and mesodermal 
cell-groups fuse as the crest ganglion grows downward. In the ventral 
region it meets the mesoderm of the mandibular arch. 

Embryology of Tadpole and Chick 151 

It is important that the student note how the mesenchyme of the 
mandibular arch is formed by the process of growth just described. The 
mesodermal and ectodermal cells have so intermingled at the point of 
fusion that the separate cells of ectoderm and mesoderm are now indis- 
tinguishable. (Fig. 334.) 

The dorsal and superficial cells of the crest ganghon retain their 
nervous character and come into close relation to the large placode close 
to them ; then the superficial sensory portion of the placode disappears. 
The deep or ganglionic portion not only enlarges, but divides into two 
parts. The anterior portion becomes the ophthalmic ganglion of the 
ophthalmic branch of the V nerve. The fibers of this branch grow 
cephalad through the dorsal head region, and also medially connect with 
the medulla oblongata. 

The posterior portion of the placode ganglion fuses with the crest 
ganglion to form the Gasserian ganglion or trigeminal ganglion. 

It is from the cells of the trigeminal ganglion that fibers arise which 
run to the medulla on the dorsal side. These fibers form the sensory 
root of the V nerve. 

Then, too, there are fibers which grow out from the ganglion to pass 
to the surface of the head to form the cutaneous branch of the V nerve, 
while the fibers, which pass in front of and behind the mouth, are called 
the mandibular and maxillary branches respectively. 

All of these branches, as well as those from most of the branchi- 
omeric nerves, can be seen before the opening of the mouth. 


Both of these nerves are derived from the acustico-facialis crest 
ganglion and the placode associated with it. The VII nerve is connected 
with the hyomandibular cleft, while the VIII nerve is a purely sensory 
(auditory) nerve, and so not one of the branchiomeric series. 

The greater portion of the crest ganglion, as with the V nerve, 
contributes to the mesenchyme of the hyoid arch, although the nervous 
portion of the crest ganglion is more extensive than that of the V nerve, 
which is due to the fact that a greater portion of the original ganglion 
retains its nervous function. 

The superficial, or nervous, character of the placode does not 
disappear in this case, but keeps on becoming larger, after which it sinks 
below the surface of the head and invaginates to form the auditory sac. 
(Fig. 334.) 

The deep placode ganglion cells which are in connection with this 
sensory epithelium remain in contact with the sac to form the root of 
the VIII nerve. 

The remaining portion of the placode ganglion joins with the nerv- 
ous portion of the crest ganglion to form the ganglion of the VII nerve. 
It is from this ganglion that fibers pass to the medulla and to the hvoid 

152 The Embryology of the Frog 

arch and associated regions, to form the hyomandibular and palatine 


The remaining visceral clefts, that is, the first to fourth clefts, or 
third to sixth visceral arches, are associated with the IX and X nerves. 

The IX nerve is limited to the first gill cleft alone, but the X nerve 
is associated and distributed to the others. It is to be considered a 
compound nerve, as it is made up of several branchiomeric nerves. 

The large posterior part of the neural crest in the head region is the 
portion associated with the IX and X nerves. 

Its growth is much like that of the V nerve, though it does not 
assist in forming so much mesenchyme. 

The superficial sensory portion of the placode of the IX nerve 
disappears, and its ganglionic portion is only slightly related to the crest 

Posterior to this, the larger placode of the X nerve appears simulta- 
neously, and passes through similar stages ; but in this case there is a 
more extensive fusion between it and the nervous portion of the crest 

The fibers from the IX and X ganglion pass out together tO' the 
medulla as a single root. The anterior cardinal vein partially separates 
the IX and X ganglion. 

The fibers which pass out from the IX nerve portion of the ganglion 
are practically all placodal in origin, and pass to the first branchial cleft ; 
while the fibers coming from the mixed ganglion of the X nerve are 
connected with all the remaining clefts. 

It is well to pay considerable attention to the X nerve, as it is one 
of the most important nerves in the body, and is connected with many 
important structures. 

It is from the X ganglion that other processes than those just men- 
tioned, also grow. A considerable tongue of cells grows out posteriorly 
to form the sense organs of the lateral line, shortly to be discussed; while 
the fibers, which are to become the lateral line nerves, accompany this 
tongue (Fig. 340). These latter fibers are present only during the 
tadpole stage. 

Then there are branches which pass to the thoracic and abdominal 
organs to form the visceral branch of the X nerve. 

As sensory nerves pass to a general center, and motor nerves pass 
from a center to some outlying region, it is well to appreciate how some 
of the nerves mentioned above come to be mixed, that is. how they 
happen to have both sensory and motor fibers running along side by side. 

The motor fibers of the branchiomeric series do not arise by 
separate roots (Fig. 336) as do the sensory, but from neuroblasts in the 

Embryology of Tadpole and Chick 


walls of the medulla oblongata which send out processes called axons, 
which leave the medulla in close association with the sensory roots 
already described. These are then distributed with the branches passing 
posterior to the gill clefts. 

The III cranial nerve is the first of the remaining III, IV, and VI 
to appear, although all three of these form later than the ones discussed 
above, that is,. they form when the tadpole is five to six millimeters in 

The III is called the oculo-motor, the IV the trochlear, and the VI 
the abducens. All are motor nerves, which innervate the muscles of the 

The I cranial nerve is the purely sensory olfactory nerve, and the II 
nerve is likewise a purely sensory nerve, namely, the optic. 


These nerves, unlike the cranial nerves, are related to the somites, 
and not to the visceral clefts, and no placodes arei connected with 

The two most anterior myotomes 
do not have spinal nerves connected 
with them, and the myotomes soon 
disappear ; but the segments formed in 
the neural crests, posterior to the head 
region (with the exception of the two 
just mentioned), have cell processes 
grow out into the cord to form the 
dorsal root of the spinal nerves, while 
others grow away from the cord to 
form the peripheral strands which are- 
distributed to the skin and other 
sensory surfaces. 

The ventral root of the spinal 
nerve is formed by outgrowths, or 
axons, from the neuroblasts on the 
ventral side of the cord, and appear 
when the tadpole is about four milli- 
meters in length. These then meet the 
dorsal root a little distance beyond the ganglion, and pass partly to the 
mesodermal myotomes and partly to the sympathetic system. 


When the tadpole is about six millimeters in length, one may see a 
slight collection of cells on the spinal nerves at about the level of the 
dorsal aorta. From our study of the sympathetic system in higher 
animals, we assume that these cell-groups are composed of elements from 
the spinal ganglia, and from some of the posterior cranial ganglia. 

Schematic arrangement to show the 
composition of the central nerve roots in 
shark fins. The motor fibers run to the 
muscles, and each motor spinal root is 
made up of the fibers of three spinal seg- 
ments. NI, II, III, IV, V, Neuromeres; 
N2, N3, N4, corresponding motor roots; 
M1-M5, Myomeres; 1-10 Divisions of 
Myomeres. (From Rabl.) 


The Embryology of the Frog 

The cells themselves migrate ventro-medially to form a pair of 
longitudinal sympathetic cords, along each side of the dorsal aorta. It is 
from these cords, then, that processes grow back to the spinal ganglia to 
form the rami communicantes, as well as outwardly to the various organs 
and surfaces. 

Other fibers from other spinal ganglia grow out and follow the paths 
thus laid down for them, while cells from the sympathetic cord probably 
also migrate to form the large sympathetic ganglia found in close con- 
nection with the large blood vessels, and the thoracic and abdominal 
viscera. The fact of the matter is that the sympathetic nervous system 



Fig. ZZT. 

A. Sympathetic nervous system of the Frog, ao, aortic arch; 
ao.c, common aorta; isch, ischial nerve;, communicating 
branches between the spinal and sympathetic nerves; sy, the two 
branches of the sympathetic system; 1-10, spinal nerves. (After 

^ B. One-half of a transverse section of Ammocoetes, in the head- 
region. Schematic, ao, aorta; hr.a., branchial branch of nerves; ch, 
notochord;, ganglion anlage which develops where the 
epibranchial placode forms; ect, ectoderm; ent., entoderm;, 
lateral placode;, medial placode; med.obl., medulla oblongata 
(hind-brain) ;>ni, myotome; s.c.a., subcutaneous branch of the epi- 
branchial nerve placode; sp.a., spinal branch running inward; s.pL, 
lateral plate of mesoderm; symp.g., sympathetic ganglion. (After von 

of the frog has not been worked out with any degree of thoroughness, 
and we can only suppose many things from our knowledge of other forms 
where more is known of this system. 

The ganglion of the III cranial nerve is sympathetic in character, 
as other cranial nerves may be, but this must be left for future workers 
to demonstrate. 

Embryology of Tadpole and Chick 


THE EYE (Fig. 338) 

The general method of the eye formation is quite similar to that 
already described in the case of the chick. 

As the outer free rim of the optic cup draws together, it leaves a 
small opening which is the rudiment of the pupil. At this time we can 
distinguish the inner and outer layer to the cup, and a central cavity. 
These are the beginnings of the true retinal layer, the pigment layer, and 
the posterior chamber of the eye, respectively. 

A choroid fissure is formed, just as in the chick. 

The lens forms as a thickening of the ectoderm opposite the pupil, 
but this thickening involves only the nervous layer of the ectoderm. 
It develops quite like the ectodermal placodes in the formation 
of the cranial nerves. In fact, the lens placode lies immediately anterior 
to the placode of the V cranial nerve. About the time of hatching, the 
lens has formed a prominent rounded thickening entirely cut off from 
the ectoderm. 

Fig. 338. 

Section through the eye of a tadpole at the time when the operculum is 
forming, much magnified, ch, choroid; cor, mesoblast cells which will give 
rise to the cornea; dm, Descemet's membrane; ep, pigmented external 
epithelium; L, lens; mes, branched mesoblast cells; op.n., optic nerve; 
pch, pigmented epithelium of the choroid; R, retina; rd, rods and cones 
pulled away from the pigmented epithelium of the choroid by contraction 
of the preparation; vit, cells of the vitreous humour. (After Bourne.) 

This spheroidal mass hollows out, although it again becomes solid 
by the cells on its inner side elongating, while the outer side remains 
as a thin epithelial layer covering the distal surface of the lens. Then, 
as the pupil narrows, the lens comes to lie just within the opening of 
the cup. 


The Embryology of the Frog 



dr. med. 



Loifti'laj^iKcncs hccial cxt^i^le. . 

JcLLolfSoyt's CHda"^. 

iu/oer/of y>n».tiUx^f P''oc9s&. 

r p^fcZis. 

Fig. 339. 
Reconstruction of smelling apparatus o£ Frog and three transverse sections (on 
the right) of tadpole through the regions marked A A, B B, and C C., 
glands on the medial side; lat, lateral view of the olfactory cavity; lat-u, com- 

Embryology of Tadpole and Chick 157 

In the chick, and in fact in all vertebrates except the Teleosts, the 
lens forms as a hollow vesicle caused by the surface ectoderm invagi-- 
nating. The choroid fissure of the optic cup closes a day or two before 
hatching. The closing begins opposite the pupil. 


The auditory placode appears just as the neural folds close. These 
placodes then become depressed below the surface of the head and 
invaginate to form the auditory sac or otocyst. 

This sac at the time of hatching has become completely closed and 
separated from the ectoderm from which it arose. Therefore, it comes 
to lie in close relation to the lateral surface of the medulla. 

The superficial layer of ectoderm continues to remain as a covering 
of the region where the placode invaginated. The wall of the auditory 
sac is but one cell in thickness, except in the medio-ventrai region. It 
is in this region that the ganglionic part of the placode is located. There 
is a small finger-like outgrowth from the sac, which extends dorsally 
from the medio-dorsal region. This is to become the endolymphatic 

There is little change in the ear region from this time to the opening 
of the mouth, that is, until the tadpole is ten to twelve milHmeters in 
length. Then development seems to begin again. 

The remaining complicated changes in the formation of the inner 
divisions of the ear are beyond the scope of this book. The VIII cranial 
nerve connects with the ear. 


The olfactory organs appear cj^uite early, in fact before the brain 
closes (Fig. 339, na). A pair of ectodermal thickenings appears on each 
side of the head just above and anterior to the future mouth region. 
Again, only the deeper nervous layer is involved, as with the formation 
of the ear; only in this case, the superficial layer of ectoderm does not 
remain as a covering but disappears entirely. The thickenings them- 
selves lie immediately anterior to the lens placodes, and are called 
olfactory placodes. 

The placodes invaginate, forming the olfactory pits which are later 
to become the true nasal cavities (Fig. 339). A few cells from the inner 
surface of the olfactory placode become detached and come into 
communication with the surface of the fore-brain to form a sort of crest- 

munication of the lateral and lower region of the olfactory cavity; md.h., wall_ of 
the mouth-cavity; ma, external nares; o, upper region of the olfactory cavity; 
olf, olfactory nerve; o-ti, communication between the upper and lower regions of 
the nasal cavities; u, lower region of the nasal cavity. (After Bancroft.) 

D, Frontal section of human fetus of 29 mm. (After Tourneaux.) 

E, Sections through the nasal region. Bone black and cartilage^ dotted, ds, den- 
shelf; g, Jacobson's glands; j, Jacobson's organ; n, main cavity of nose; oj, 
opening of Jacobson's organ; t, tooth-germ. (After Schimkewitsch.) 

158 The Embryology of the Frog 

ganglion. It is from these cells that the sheath cells of the true olfactory 
nerves (I cranial nerves) seem to be formed. The olfactory nerves 
themselves form from the sensory cells of the olfactory placode. 

The olfactory pits are often said to form the anterior nares, while 
the openings from the anterior nares into the mouth cavity are called 
the posterior nares, internal nares, or choanae, all three terms meaning 
approximately the same thing. 

The olfactory development is quite complicated and cannot be 
worked out in all details by the student in an elementary course such as 
this, but it is important that the main features be understood, so that 
light may be thrown upon later studies. 

The epithelial lining of the olfactory pits forms a cavity on the 
dorsal side soon after hatching. This cavity then closes to form a 
separate dorso-lateral lobe, which disappears entirely as metamorphosis 

During metamorphosis, various thickenings and outpocketings 
appear in the olfactory organ, and there is a sharp bend in the main axis. 
The most important of the outgrowths is an extension from the ventral 
side of the olfactory chamber, where a solid mass of cells proliferates. 
This extended portion then acquires a cavity, grows rapidly, and turns 
transversely toward the medial side. This structure is to become 
Jacobson's organ (Fig. 339). A large glandular mass develops upon the 
medial end of this organ. 

Opposite Jacobson's organ another growth appears which is non- 
nervous, that is, is not lined with nerve cells. This becomes a large sac, 
and the cavity of the sac is then added to the olfactory chamber. Still 
another growth appears anteriorly, close to the base of the olfactory 
duct. It is into this latter structure that the duct from the lachrymal 
glands enters. Still later (about the time of metamorphosis) a dorsal sac 
grows out from the medial and posterior walls of the tube. 

During the late metamorphosis, the axis of the olfactory organ is 
sharply bent by the shifting of the internal nares, and other glands 
appear as outgrowths, both in the olfactory chamber and in the posterior 
walls of the internal nares. 


In all gill-bearing animals, and in all animals that have gills at any 
time during their development, such as the frog, a series of sensory 
organs develop, which are known as lateral line organs. These organs 
vary to a very considerable extent, but three or four of them are rather 
constant. These are: (1) the supraorbital line, which runs forward from 
the ear region over the eye to the tip of the snout. Twigs from the 
ophthalmic branch of the VII cranial nerve innervate it. 

(2) The infraorbital line, which also runs from the ear region, but 

Embryology of Tadpole and Chick 


passes under the eye to the snout. It is innervated by the buccalis nerve 
which is a branch of VII. 

(3) The hyomandibular line, which runs along the jaw and 
operculum. This is innervated by twigs from the mandibularis externus 
and finally, (4) the lateral line proper, which may be double. This 
extends back to the tail on both sides of the animal, being innervated 
by twigs from the lateralis, a branch of the X cranial nerve. 

Fig. 340. 

The development of the lateral line organs in R. sylvatica. A. Part of a 
frontal section through the level of the notochord of a 3.3 mm. embryo. B. Part 
of a transverse section through the vagus region of a 4 mm. embryo. C. Part 
of a frontal section through a 4 mm. embryo of R. virescens. D. Section through 
the lateral line organ of a 15.5 mm. larva of R. sylvatica. a. Auditory vesicle (in 
A, its rudiment); b, basement membrane of epidermis; ch, notochord; g, gut; gV, 
trigeminal ganglion, of V cranial nerve; gVIII, acoustic ganglion of VIII 
cranial nerve; gX, vagus ganglion; gXl, ganglion of the lateral nerve (branch of 
the vagus); i, intersegmental thickenings of the epidermis (ectoderm); /, rudi- 
ment of lateral line nerve; Ip, lateral plate of mesoderm; my, myotomes; n, inner 
or nervous layer of epidermis (ectoderm); nc, nerve cord; p, pigment in epi- 
dermis; si, inner sheath cells of lateral line organ; sn, sensory cells of lateral 
line organ; so, outer sheath cells of lateral line organ. E, Lateral line canals and 
their nerves in a pollack (a fish belonging to the cod group). Canals and brain 
are dotted while the lateral nerves are black, b, buccalis ramus of the VII nerve; 
dl, dorsal ramus of lateralis of X nerve; h, hyomandibularis nerve; hm., hyomandi- 
bular line of organs; io, infraorbital line; 1, lateral-line canal; n, nares; o, olfactory 
lobe; op, operculum; os, ophthalmicus superficialis nerve; soc, commissure which con- 
nects the lines of both sides ; so, supraorbital line of organs ; st,^ supratemporal part of 
lateral line vl, ventral ramus of lateralis of X nerve; x, visceralis portion of X 
nerve. {A to D, from Harrison; E, from Kingsley after Cole.) 

160 The Embryology of the Frog 

There may be also a supratemporal line, connecting the systems of 
both sides and extending across the posterior portion of the skull from 
one side to the other. 

These lateral line organs sink beneath the skin, and usually degen- 
erate in water-forms of animals as soon as the animals are ready to live 
on land. In a few cases, such as Tritons, Amblystoma, etc., the organs 
are said to reappear when the animals return to water to deposit their 

Various functions are assigned to these sensory Hne organs, but 
none has been clearly demonstrated. Probably they assist in recognizing 
differences in the vibrations of the water and may permit the animal to 
determine currents. They have been called a "sixth sense." 

In the frog tadpole, the sense organs of the lateral line are derived 
from the placode of the X cranial nerve. The ramus lateralis of the X 
nerve innervates the organ. 

When the embryo is about four millimeters in length, a small 
dorso-lateral section of the vagus ganglion is separated from (though 
lying close to) the ectodermal placode. The placode then begins to 
elongate posteriorly, while the deep cells proliferate rapidly to form a 
long, narrow tongue, which then pushes through the epidermis just 
outside the basement membrane. This tongue reaches as far back as 
the tip of the tail by the time of hatching. 

Along this line, groups of cells form at intervals, each group repre- 
senting the beginnings of a definite sense organ of the lateral line. In 
each group there are a few central sensory cells surrounded by a layer 
of enveloping cells. These groups then push up through the epidermis 
to the surface of the body, and the sensory cells develop hair-processes. 

There are other tegumentary sense organs developing in definite 
rows on the head as well as dorsally from the mid-line to form those 
mentioned above, which are innervated by twigs from the VII, IX, and 
X cranial nerves. 

All of the lateral line organs disappear, however, when the tadpole 
becomes an adult frog. ._^ 



yi FTER the splitting of the mesoderm and notochord from the ento- 
/\ derm, the wall of the digestive tract is but one cell in thickness, 
-* ^except in the region of the mid-gut, where the yolk-mass is very 

The stomodaeum, already discussed, is a shallow depression just 
below the olfactory and fore-brain region. This, by the time of hatch- 
ing, while still very shallow, has its floor come to He in contact with the 
wall of the fore-gut. The region in which these fuse is called the oral 
plate (Figs. 301, I, and 329, Pharyngeal membrane). It is at this point 
that the mouth forms a few days after hatching. The oral sucker (Fig. 
337) forms just below the stomodaeal invagination. 

The margins of the mouth form mandibular ridges which become 
drawn out as an upper and lower lip, the lower being the larger and 
freely movable. (Figs. 316, 317, 318.) 

Strands of cells from the deep layer of the epidermis push toward 
the surfaces, and as each cell arrives at the surface it becomes cornified 
in a so-called "tooth." The upper lip has three rows of these ''teeth," 
while the lower has four rows. All of these are lost when the tadpole 
assumes its adult shape. In the adult form, true teeth and jaws form. 

From the fore-gut the following structures are derived (Figs. 328, 
329, 337) : 

The Pharyngeal Cavity, the large expansion in the anterior portion 
of the fore-gut. 

The Oesophagus, that portion of the fore-gut narrowed immediately 
dorsal to the yolk. 

The Visceral Pouches, seen as vertical solid foldings extending to 
the surface ectoderm. Six pairs of these (compare this number with 
those found in the chick) develop, increasing in size and importance 

The Visceral Arches (not to be confused with the pouches) are the 
vertical rods of mesoderm lying between the pouches. 

The hyomandibular pouch is the name given the first pouch. 

The Mandibular arch is the first arch, lying in front of the hyoman- 
dibular pouch, or in other words, between the hyomandibular pouch and 
the mouth. 

The I to V Branchial, or gill pouches, are the remaining ones run- 
ning posteriorly from the hyomandibular. The hyomandibular is the 
first in point of position, but bears a separate name. The numbers thus 
begin with the true second pouch. 


The Embryology of the Frog 

The Hyoid Arch is the arch lying between the hyomandibular and 
I branchial pouches. 

Branchial Arches, or Gill Arches, are the names given to the 
remaining arches. 

The Branchial Clefts, or Gill Clefts, are the names given the various 
pouches after they have opened externally to the surface of the ectoderm, 
and internally into the pharynx. The second and third clefts are the 
earliest to become perforated, while the hyomandibular pouch does not 
become perforated at all, but disappears shortly after the first perfora- 
tions occur. 

The Eustachian tube 
or Tubo-tympanic cavity 
of the ear, forms from the 
dorsal wall of the hyo- 
mandibular pouch. 

The External Gills 

(Figs. 337, 341), which 
appear just before hatch- 
ing, are small outgrowths 
from the outer surfaces 
of the dorsal ends of the 
first and second branchial 
(second and third vis- 
ceral) arches. A small 
external gill also appears 
later on the third bran- 
chial arch. The two an- 
terior gills grow rapidly 
and form large lobed 
processes by the time the 
mouth opens. They become vascular and form the first breathing or 
respiratory organs of the tadpole. The posterior pair remain in a much 
more undeveloped state. 

The Operculum (Fig. 341) is the name given to the covering of the 
external gills. It makes its appearance before the mouth opens, growing 
out from the posterior borders of the hyomandibular arches. These 
outgrowths extend backward so rapidly that by the time the anterior 
gills have reached their maximum size, the operculum covers them in 
what is called the opercular cavity. 

The right opercular fold becomes fused with the body, while the left 
remains partly open to form the opercular tube or spiracle. 

The Internal Gills form as tiny elevations on the postero-external 
faces of the branchial arches, just as the gill clefts are perforated. A 
thin layer of ectodermal cells covers them, just as it does the external 

Fig. 341 

Ideal diagrammatic transverse section through a 13 mm. 
frog tadpole, showing the first gill arch. 

The right side shows the external gills (a.k.) extending 
through the gill opening which is almost closed. As the 
opening closes the gills draw inward. On the left side the 
gill opening is wide open with the external gills showing 
themselves free, a.b., the first gill artery; a.k., external 
gills; ao, aortic roots; and, otic capsule; c.e., external caro- 
tid; d, pharynx; s, lumen of the intestine ;/i/^r, anlage of 
the filter apparatus (gill rakers); h, heart; i.k., internal 
gills; nr, anlage of the neural canal; op, gill operculum; 
v.b.c, common branchial vein; v.b.e., external branchial 
vein; v.b.i., internal branchial vein. (After Maurer.) 

The Digestive Tract 




gills. These gill coverings become doubled on the first three branchial 
arches and remain a single row on the fourth branchial arch. 

The Gill Filaments are the branched processes of the gills which 
become quite vascular and, as they project into the opercular cavity, 
permit the exchange of gases from the respiratory current of water 
entering the mouth, which current then passes through the gill clefts 
and the opercular tube. 

The Velar Plates and 
Gill Rakers (Fig. 341, 
filtr) form as tiny folds in 
the pharyngeal region 
which can be best under- 
stood by a study of the 
cuts. (The velar plates 
are the smooth inturnings 
of the floor of the pharynx 
while the jagged tooth- 
like foldings are the gill 

The structures men- 
tioned above are prac- 
tically lost when the frog 
reaches adultship, 
although portions of these 
structures are used in 
building and forming 
other structures which 
will be discussed shortly. 
The thymus body 
(Fig. 342) appears just 
prior to hatching. It is a 
solid internal proliferation 
from the epithelium of the 
upper side of the first 
branchial pouch (second 
visceral, or hyobranchial 
pouch). There is alsO' a 
similar outgrowth from the hyomandibular pouch, but this disappears in 
a short time. The outgrowth which is to become the thymus grows 
slowly and separates entirely from the wall of the pouch when the 
tadpole is about twelve millimeters in length. 

Both permanent and transient thymus bodies lie close to the outer 
surface of the head, immediately behind the auditory capsule and the 
articulation of the jaw. 

Epithelioid bodies are similar to the thymus glands. These are 

Fig. 342. 

I. Diagram of the branchial pouch derivatives in the 
frog. (After Maurer, with Greil's modification.) eg. Carotid 
gland; e^, e^, e^, epithelioid bodies; th, thyroid body; 
tm^, tm^, thymus bodies; uh, ultimobranchial body; I-VI, 
first to sixth visceral pouches. (/, hyomandibular; I I-VI, 
first to fifth branchial pouches.) 

II. Diagrams of the derivatives of the visceral pouches 
and arches in the frog. A. Lateral view, frog larva. B. 
Lateral view, after metamorphosis. C. Transverse section 
through gill of frog larva. D. Transverse section through 
gill region, just after metamorphosis; the gills have not 
quite disappeared, a. Afferent branchial arteries; c, carotid 
gland; d, dorsal gill remainder; e, epithelioid bodies; g, 
internal gills; in, middle gill remainder; o, operculum; s, 
suprapericardial or postbranchial body; t, thyroid body; th, 
thymus bodies; v, ventral gill remainder; 1-VI, visceral 
arches; I, mandibular arch; //, hyoid arch; II I -VI, first to 
fourth branchial arches; 1-6; visceral pouches; 1, hyo- 
mandibular pouch; 2, hyobranchial pouch; 3-6, first to fourth 
branchial pouches. (Greil's Modification of Mauer.) 


The Embryology of the Frog 

formed from the dorsal ends of the other branchial clefts and become 
lymphoid in character. 

The Carotid Glands form from the ventral ends of the anterior gill 
pouches as epithelial proliferations at about the time the internal gills 

The Pseudothyroid Body is a small outgrowth in the postero-ventral 
branchial region, apparently having no relation with the disappearing 
gill-clefts. This body disappears with the exception of traces of the 
middle and ventral portions which persist for a short time after meta- 
morphosis, after which they, too, disappear. 

The Ultimobranchial Bodies 
(also called post-branchial or supra- 
pericardial bodies) lie posterior to 
the fifth visceral pouch. They have 
formed as solid proliferations from 
the pharyngeal wall just behind the 
visceral (fifth) pouch, (fourth 
branchial pouch). These bodies are 
supposed to represent vestigial por- 
tions of a sixth pair of visceral 
pouches, although they do not ex- 
tend to the surface ectoderm. They 
separate from the pharynx and 
acquire a lumen, coming to lie on 
the floor of the pharynx in a supra- 
pericardial position. (Fig. 342.) 
The Thyroid Body is formed as 
a medial invagination from the floor of the pharynx just a short time 
before hatching. It forms as a solid rod of cells, but a few days after 
the opening of the mouth it forms a pair of bodies which grow rapidly 
and become very vascular. The thyroid body has no genetic relation- 
ship to the branchial structures nor do any of the following structures 
possess such relationship. 

The Lungs develop just before hatching, as a pair of solid prolifera- 
tions from the ventral wall of the posterior portion of the fore-gut, just 
between the yolk-mass and the heart. The cavities begin to form in the 
proximal region'. 

The Laryngeal Chamber (Fig. 332, B) is formed by the wall of the 
fore-gut between and around the lung diverticula which become 
depressed and form a groove, which then (at least partially) constricts 
off from the alimentary tract. 

The Glottis is the opening which remains in the laryng-eal chamber 
as it constricts. 

The Tongue appears just before metamorphosis as an elevation in 

Fig. 343. 
From a model of the duodenum and the 
primary evaginations of the liver and pancreas 
in a 5 mm. sheep embryo. D.pan., Dorsal 
pancreas; Du., duodenum;, ductus 
choledochus;, gall bladder; H.du., hepatic 
duct. (After Stoss.) 

The Digestive Tract 165 

the floor of the anterior portion of the pharynx, just behind the thyroid 
region. There is a glandular depression directly in front of the elevation 
which is carried forv^ard by the anterior growth of the tongue, so that 
the glandular portion becomes the tip of the tongue. 

The Liver, one of the earliest diverticula of the alimentary tract, 
appears e^en before the embryo itself has begun to elongate. It Hes 
beneath the yolk mass and develops from the ventral portion of the fore- 
gut, just posterior to the heart. A group of scattered mesodermal cells 
lies between liver and heart, which will soon be added to the anterior 
wall of the liver rudiment. The anterior portion of the liver becomes 
folded after hatching. 

The Gall-bladder is formed as a postero-ventral extension of the 
liver diverticulum which becomes more or less separated from the 
anterior portion of the liver. 

The Bile-duct (Fig. 332) is the original opening of the liver 
diverticulum into the alimentary tract from which it grew. 

The Pancreas (Figs. 343, 293) develops in close proximity to the 
liver diverticulum from three separate rudiments, a dorsal and two 
ventral. The dorsal rudiment is a soHd outgrowth from the dorsal wall 
of the fore-gut. A complete separation between outgrowth and origin 
soon takes place. 

The right and left ventral rudiments grow out of the fore-gut at the 
posterior margin of the bile-duct. These retain their connection with 
the gut, enlarge, and after passing around the bile-duct, fuse together 
in front of it. The dorsal portion later also fuses with this fused right 
and left portion and connects with the gut by the pancreatic duct. At 
this period, the pancreatic duct forms the boundary between the fore-gut 
and the mid-gut, although later the pancreatic duct comes to lie 
within the margin of the bile-duct. Oesophagus and stomach develop 
quite as they do in the chick. It will be remembered that the oesophagus 
closes for a time in the chick. It does likewise in the frog just after 
hatching, when the tadpole is about eight millimeters in length. 

The oesophagus, however, again opens just before the mouth is 
formed, that is, when the tadpole is about ten or eleven millimeters 

The stomach is at first longitudinal, but it soon bends, and comes 
to lie transversely. This position is a matter of importance when nerves 
are to be traced, for the nerves and blood vessels which are paired and 
known as right and left will now be dorsal and ventral, depending upon 
the side toward which the caudal end of the stomach turns. 


As with the chick, the mid-gut is confined to that more or less 
central space where the yolk continues being absorbed and converted into 
other substances in the growing embryo and tadpole. After hatching. 

166 The Embryology of the Frog 

the yolk is absorbed very rapidly, some of the yolk-cells becoming the 
epithelial lining of the growing intestine. The intestine is bent into a 
transverse loop, called the duodenal loop, with its proximal end at the 
posterior portion of the stomach. In fact, the intestine grows to almost 
nine times the length of the tadpole's body, but is shortened later to 
about one-third the body-length by the time metamorphosis takes place. 
The Hypochordal rod is a structure which forms from the mid-gut 
as a medial ridge along the surface of its entodermal wall just under- 
neath the notochord, although it has no relation to the notochord. It 
appears when the tadpole is three or four millimeters in length, and 
extends both cephalad and caudad, coming to lie free of the gut wall 
when the tadpole reaches a length of 4.6 millimeters. Finally, it can 
be seen as a caudal extension from the dorsal pancreas posterior through 
the tail. The rod itself is narrow, being only two or three cells in 
diameter. Just before the opening of the mouth, it breaks into pieces 
and disappears entirely. 


It is in this region that the neurenteric canal is found (Fig. 328), 
as well as the proctodaeum. The terminal end of the hind-gut, which 
is to become the rectum, fuses with the proctodaeum, and the anal open- 
ing perforates the thin sheet which has separated these two structures. 
This fusion and perforation takes place at about the time the tail begins 
to elongate, namely, when the embryo is about four millimeters in length. 
It is the proctodaeal region of both tadpole and frog which forms the 
cloaca into which rectal, excretory, and reproductive ducts enter. The 
bladder forms as a ventral outgrowth from the cloaca just before 

The Post-Anal gut is formed by the true hind-gut which remains 
within the embryo body proper as the tail continues growing. The nerve 
cord and notochord, however, are carried along in the growing tail so 
that the neurenteric canal is drawn out caudad. This neurenteric canal 
is then cut off from the nerve cord, but for a short period its antero-ven- 
tral portion opens into the rectum, and it is this which is known as the 
post-anal gut. This gradually closes, although a strand of cells can still 
be seen in the region at the time of hatching, extending nearly to the tip 
of the tail. It finally disappears altogether. 



A LL THE remaining systems to be described are intimately related 
/\ to the mesoderm. 
^ ^ The mesodermal region in the chick, posterior to the head, 
divides into block-like segments by the formation of connective tissue 
septa which form at right angles to the long axis of the embryo. So,' 
too, in the frog embryo and tadpole, the mesoderm divides into block-like 
segments. These are the somites. And, just as with the chick, so with 
the frog embryo, the mesoderm divides into an outer somatopleure and 
an inner splanchnopleure layer with an open space between them, known 
as the myocoele. 

That portion of the somites which lie directly on each side of the 
notochord, is known as the segmental, or vertebral, plate, while the por- 
tion extending laterad from this segmental plate is called the lateral 

The lateral plates are, therefore, merely direct continuations of the 
segmental plates. However, as the somites divide off into blocks, the 
vertebral plates become thickened, and not only is the myocoele closed 
which lies within them, but a constriction along the long axis of the 
embryo separates the vertebral plate from the lateral plate. 

As there are no somites in the head region, the mesoderm lies in 
the form of scattered groups of mesenchymal cells in the head and 
pharyngeal regions. 

The somatopleure and splanchnopleure are not of equal thickness. 
The outer somatopleure is only one cell in thickness, while the inner 
splanchnopleure is much thicker. Therefore, the coelom which lies 
between these two layers, lies much closer to the outer portion of the 
embryo than to the inner portion. 

The one cell-layer, which forms the outer somatopleure in the region 
where the somites have formed, is naturally segmented, as that layer is 
an actual portion of the somite proper. These one cell-layered segments 
of somatopleure are now called dermatomes or cutis plates, because they 
will soon join with the ectoderm lying immediately above them to form 
the outer wall of the embryo. 

The segments of the splanchnopleure, which lie toward the center 
of the embryo, are called myotomes, or muscle plates, because it is from 
these that the muscle cells will form. In fact, the formation of the 
muscle fibrils can already be observed when the embryo is scarcely five 
millimeters in length. 


The Embryology of the Frog 

It is the thickening of the myotomes which obliterates the myocoele 
quite early. 

From the lower ends of each myotome, that is, from those regions 
lying ventral and close to the mid-line, cells proliferate and move down- 
ward beneath the notochord as well as upward between the notochord 
and the myotome. These proliferations form the sclerotomes which are 
to become the cartilaginous vertebral column. 

Very early, that is, when the tadpole is only about five millimeters 
long, the somite proper separates from the lateral plates, and the 
sclerotomes separate from the lateral plates. 

Immediately after this separation, there are ventro-lateral out- 
growths from both myotomes and dermatomes. Those from the 
myotomes become the ventral musculature and extend into the limbs as 
voluntary muscles, while those from the dermatomes break up into 
groups of mesenchymal cells, some of which become connected with the 
inner surface of the ectoderm, and form the dorsal covering of the 
embryo, while some pass between the myotomes to form the connective 
tissue septa, also called myocommata. (Fig. 423.) 

There are thirteen pairs of somites formed in the trunk of the frog, 
the two most anterior pairs disappearing in the adult. The full-fledged 
frog, therefore, has but eleven definite segments. The two anterior 
somites become part of the occipital region of the head. In the tadpole, 
there are many more somites than the amount stated above, but with 
the loss of the tail and the conversion of tadpole into frog, these are lost. 

The following table will not only summarize the history of the 
somites and spinal nerves, but if carefully, studied will show how the 
adult vertebral musculature which connects the posterior half of one 
vertebra with the anterior half of the next succeeding one, receives its 
innervation from more than one spinal nerve. A clear understanding 
of this will help the student very materially in his comparative anatomy. 
(See Fig. 336 also.) 



(From Kellicott, after Elliot) 

Elements in 







Regrion of 


Absent (Disappears at 
formation of limbs) 

Root of vagus nerve 



Absent (Disappears at 
formation of verte- 

No nerve. Ganglion in 
embryo only 



Ganglion and Nerve in 

Absent in adult 

The Mesodermal Somites 


1 Vertebra 




Spinal Nerve 


2 Vertebra 




Brachial Plexus 

3 Vertebra 



4 Vertebra 




to body wall 

5 Vertebra 



6 Vertebra 



7 Vertebra 




Sciatic plexus 

8 Vertebra 



9 Vertebra 



Part of 




to pelvic region 

Even before the lateral plates are separated from the somites, there 
is a tendency to segment in the lateral plate itself, close to the somite. 
It is in this region that the excretory system is to be formed, and so 
these portions are called nephrotomes, or the intermediate cell mass. 
(Fig. 268.) 

This mere trace of segmentation in the lateral plate lasts a very 
short time. In fact, the lateral plate itself never segments. 

The cavity within the lateral plates is the true coelom. Just as in 
the chick, when the lateral plates extend further and further ventrally, 
they finally meet in the midline and fuse, to form a single coelom where 
there had been one on each side before. 

This ventral fusion forms the ventral mesentery which soon disap- 
pears except in the heart region. The paired coelomic cavities also come 
together dorsally between the notochord and the digestive tract, and 
fuse to become the dorsal mesentery. This remains as a suspensory 
arrangement for the gut (Fig. 293). The gut itself later sinks more and 
more ventrally and the mesentery is pulled ventrad with it. The blood 
vessels supplying the digestive tract then grow downward through these 
two layers of mesentery. 



IMMEDIATELY cephalad to the liver, just ventral tO' the hinder part 
of the pharynx (Fig. 328), is the heart region. Here the somatic and 
splanchnic layers of mesoderm have separated by a wide cavity which 
is to become the pericardial cavity (Fig. 344), continuous for a time with 
the general body cavity, but later closed off. 

nd cartl. mef carcl.v. 

f n I cant, pt r i ard.' 

Fig. 344. 

Cross sections of frog tadpole. A and B in the region of the anterior and 
posterior portions of the heart respectively; C, through a more or less mid- 
section of a more advanced tadpole, for comparative purposes, ect, ectoderm; 
end.card., endocardium; ent, entoderm; mes.card.d. and mes.card.v, dorsal and 
ventral mesocardium; par, somatic layer of mesoderm (somatopleure) ; per. card., 
pericardial cavity; vise, visceral layer of mesoderm (splanchnopleure). (From 

Both pericardial wall and muscular wall of the heart are derived 
from the lateral plate mesoderm, while the endothelium — the inner lining 
of the heart — arises from scattered mesodermal cells lying between the 
splanchnic mesoderm and the digestive tract. These cells can be seen in 
the two somite stage. 

From Figure 344 one can get a good understanding of how the 

The Circulatory System 171 

lateral plates extend beneath the pharynx and fuse in the midline to form 
the ventral mesocardium. 

The splanchnic layer of the pericardium becomes folded dorsally so 
as to enclose the endothelial cells which have formed a more or less 
short tubular arrangement. Then the folds meet and fuse dorsally to 
form another tube on the outside of the endothelial tube. These latter 
tubes thus enclose the endothelial tubes and are connected with the 
dorsal wall of the pericardial cavity. The connection forms the dorsal 
mesocardium. The outer tube forms the muscle of the heart walls. 

Much difficulty will be avoided in later studies if, after a review of 
the chick's circulatory system, the following account of the heart be 

The paired endothelial tubes fuse together cephalad to form the 
bulbus aortae. Then there are outgrowths which form the beginnings 
of the truncus arteriosus or ventral aortae. The endothelial tubes do 
not fuse entirely in the posterior region, and are not alike on each side. 
The right one is bent and forms the beginnings of the ventricle and the 
right vitelline vein. The left one is slightly longer and larger in diam- 
eter in the more caudal portion. This is to become the auricle, while its 
continuation in a caudal direction will become the left vitelline vein. 
Both vitelline veins are connected directly with the yolk-mass and the 

As the embryo develops, the two endothelial tubes fuse quite like 
the letter S, after which the dorsal mesocardium disappears, so that, as 
in the chick, the heart tube is attached only at its two ends. 

The heart now comes to have its caudal end toward the left side 
and resting against the liver. This caudal portion is the region of the 
sinus venosus and the auricles. 

However, as the heart continues growing much more rapidly than 
the surrounding portions, and as it is attached only at its two ends, the 
cephalic region of the heart and the ventricle region swing downward 
and come to lie ventral in position. This naturally forces the auricle 
dorsad, and the auricle thus comes to occupy the greater portion of the 
dorsal side of the adult heart. 

Constrictions separate the heart early into two limbs, but it is only 
after the tadpole's mouth has opened that the auricle is divided into 
right and left halves by the inter-auricular septum which grows ventrad 
from the dorsal wall. The sinus venosus remains connected with the 
right auricle, while the pulmonary veins later enter the left auricle. The 
pulmonary veins can hardly be seen during the tadpole stage. 

The ventricle walls thicken, and a few days after the mouth opens, 
the bulbus aortae divide into an anterior and a posterior portion. The 
anterior portion is called the truncus arteriosus. This truncus arteriosus 
is also divided into right and left channels. 


The Embryology of the Frog 


The Lateral Dorsal Aortae, which are the first to appear, are paired 
and He dorsal to the pharynx. They are formed from a series of sepa- 
rate spaces in the mesenchyme of the head region which then connect 
to form the vessels of the cranial region. ^ 

The Dorsal Aorta (Fig. 345, ao) is the name given the lateral dorsal 
aortae as soon as they fuse medially on the dorsal side of the embryo to 
form a single vessel. This vessel extends to the caudal extremity of the 

The Aortic Arch. An aortic arch forms in each branchial arch, first 
as a little vascular space when the embryo is about 4.5 millimeters long. 

Diagrams of the heart and chief arteries of a tadpole. A, the vessels 
of a tadpole at the stage when three external gills are present; B, the 
arrangement when secondary gills are in use; C, the adult arrangement, 
a.c, Anterior commissural vessel;, anterior cerebral artery; a/., 
carotid gland; cu., cutaneous artery; d.h., ductus Botalli; ej., efferent 
branchial arteries; ht., heart; hy., efferent hyoidean artery; %., connecting 
vessel; /., lingual artery; md., efferent mandibular artery; p.c, posterior 
commissural vessel; pl.c, pulmocutaneous arch; pul., pulmonary artery; 
sys., systemic arch; tr., truncus arteriosus; v, ventricle; I-IV ., branchial 
aortic arches. (After Bourne.) 

This space connects ventrally with the truncus arteriosus and dorsally 
with the lateral dorsal aorta (Fig. 341, a.b.). It is now called an aortic 
arch. There are four pairs of aortic arches lying in the third to sixth 
pair of gall arches. The first and second aortic arches are formed in the 
mandibular and hyoid arches. 

The Afferent Branchial Arteries (Fig. 345, af). As the external 

The Circulatory System 173 

gills develop, a vessel forms dorso-laterally to the aortic arch, along the 
base of the gill to supply it. The vessel opens out of the ventral end 
of the aortic arch only to join it again at its upper end. The lower end 
of the aortic arch is then called the afferent branchial artery. 

The Efferent Branchial Arteries (Fig. 345, ef). These are but the 
dorsal ends of the aortic arches. Loops of tiny capillaries form in the 
external gill to connect afferent and efferent vessels. Later, after the 
external gills are lost and the internal ones developed, part of the aortic 
arches disappear, so that afferent and efferent vessels are connected by 
vessels of the internal gills. This causes the original aortic arch to 
become almost entirely an efferent branchial artery, while the vessel 
which connected ventral and dorsal ends of the aortic arch becomes the 
afferent branchial artery (Fig. 345). 

As the internal gills disappear when the tadpole becomes a frog, the 
lower end of the efferent branchial artery, which is the original aortic 
arch, again acquires a direct connection with the afferent branchial artery 
so that the blood again passes from truncus arteriosus to dorsal aorta. 

The gill capillaries diminish and the connection, which has thus been 
re-acquired, becomes larger and forms the adult persistent vessels of the 
branchial arches. As the -fourth branchial arch has no external gills 
developed upon it, the part described above regarding external gills does 
not apply to it. In all other respects, its blood supply is similar to those 
which do have external gills. 

The above description applies to Rana esculenta, which, although 
different in detail, is essentially alike in all species. 

The following account of the mandibular and hyoid vascular 
arrangement is for Rana temporaria: 

Hyoidean Vein. This is a small outgrowth of the lateral dorsal 
aorta which extends toward, but never reaches the vessel which repre- 
sents the aortic arch of the hyoid arch. It disappears at the time the 
mouth opens. 

At the time of hatching, there is a small outgrowth from the truncus 
arteriosus extending into the lower end of the hyoid arch, but this, too, 
disappears shortly, though it is at this time that the vestige of the 
aortic arch has already become divided into dorsal and ventral portions. 
The dorsal portion also disappears, and the ventral becomes the hyoidean 
vein. This disappears with the oral sucker. 

Pharyngeal Artery. Just before hatching, the vessels of the mandib- 
ular arch appear. Here, too, there is a vestigeal aortic arch in the lower 
portion of the mandibular arch, which soon unites with an outgrowth 
from the lateral dorsal aorta. After vmion, this vessel joins the hyoidal 

After the mouth opens, the outgrowth from the lateral dorsal aorta 
separates from the other vessels, growing forward. It is then known 

174 The Embryology of the Frog 

as the pharyngeal artery, while, as mentioned above, the hyoidean vein 
disappears with the oral sucker. 

Anterior, or Internal Carotid, Arteries (Fig. 345, car). These are 
the extensions of the lateral dorsal aortae into the head. "^ 

Commissural arteries (Fig. 345, ac). These are the two transverse 
connections between the internal carotids, which pass anterior and 
posterior to the infundibulum. 

Lingual, or External Carotid, Arteries (Fig. 345, 1.). These appear 
before the time of the mouth opening as a pair of sinuses in the buccal 
cavity. As the mouth opens, they extend backward to connect with the 
ventral ends of the efferent branchial arteries of the first branchial arch 
at the point where the carotid gland is to develop. 

Changes in Aortic Arches. As soon as the gills disappear, there 
must naturally be a great modification in the branchial aortic arches 
(Fig. 345). 

Carotid Arch. As already stated, a continuous aortic arch is 
reestablished in each of the four branchial arches when the afferent and 
efferent arteries fuse. The first branchial aortic arch, which is the third 
arch of the entire series, remains as the carotid arch. 

Systemic Arch. The lateral dorsal aorta between the carotid arch 
and the aortic arch immediately posterior to it, is reduced to a solid 
strand of connective tissue and no longer functions, so that the second 
pair of aortic arches (the fourth of the whole series) form the roots of 
the dorsal aorta and is called the systemic arch. 

Pulmocutaneous Arch. The third aortic arch (the fifth of the whole 
series) also becomes a solid strand of tissue and then disappears entirely, 
while the fourth (the sixth of the whole series) becomes the 
pulmocutaneous arch. 

Pulmonary Arteries. These appear just after hatching as small 
outgrowths from the upper ends of the efferent branchial, arteries of the 
fourth branchial arch, which then extend backward to the lung 

Cutaneous Arteries. These leave the pulmonary arteries and extend 
dorsally to spread out over the skin of both sides and back. 

Ductus Botalli. This, it will be remembered, from our study of the 
chick, is that part of the fourth aortic arch between the lateral dorsal 
aorta and the origin of the pulmonary arteries (Fig. 345). This portion 
slowly atrophies and also becomes a solid strand. Three channels are 
now formed in the truncus arteriosus by various septa which divide it 
longitudinally. The carotid arches lead from one of these channels, 
which receives blood from the left side, while another carries venous 
blood from the right side of the heart to the pulmocutaneous arches. 
The remaining one connects the systemic arches. 

The Circulatory System 



About the time of hatching, outgrowths of the dorsal aorta, juSt 
back of the pharyngeal region, extend laterally into the region of the 
pronephros or head kidney. These later become very large and form 
the vascular glomi of the kidney, traces of which remain long after the 
pronephros itself has disappeared. 

In the frog the origin of the blood itself is by no means well 
established. The student should review the work on the chick in this 
respect. All we can say in regard to the frog is that when the embryo is 
almost ready for hatching, that is, when^ it is four to 4.5 millimeters in 
length, irregular spaces appear in the mesenchyme and splanchnic 
mesoderm, which later form continuous vessels. The corpuscles may 
arise as outgrowths from the endothelial lining of the blood vessels 
themselves, or from blood-islands which form on the ventral side of the 
yolk mass, or they may possibly form from both these sources. 


Fig. 346. 

The development of the posterior part of the venous system in the 
frog. _ A. Portion of a transverse section through the posterior meso- 
nephric region of an 18 mm. tadpole. B. Diagram of the veins of a 25-30 
mm. tadpole. C. Diagram of the veins of the adult from, a, Dorsal aorta; 
c, vena cava; e, nuclei of the endothelial lining of the mesopheric sinus, 
continuous with the vascular endothelium; /, femoral vein; i, iliac vein; Ic, 
lateral mesonephric channel of the posterior cardinal vein; m, mesentery, 
tnn, mesonephros; n, mesonephric tubules; p, posterior cardinal veins (in 
C showing their original location); pv, pelvic vein; rp, renal-portal vein; 
rr, revehent renal veins; sc, sciatic vein; st, nephrostome; u, caudal vein; 
vcm, median mesonephric channel of the posterior cardinal vein; W , 
Wolffian duct; x, connection between caudal vein and the lateral meso- 
nephric channels; 1-1, part of the renal-portal vein formed from the lateral 
channel of the posterior cardinal; 2-2, part of the renal-portal vein formed 
from the median channels of the posterior cardinal vein. (After Shore.) 

THE VENOUS SYSTEM (Figs. 308, 346) 
Omphalomesenteric, Vitello-intestinal, or Vitelline Veins. These 
veins are really the first part of the vascular system to form in the 
region of the blood-islands. They are paired but not alike on both sides. 

176 The Embryology of the Frog 

They pass along the lateral surface of the yolk and liver, and enter the 
sinus venosus. In fact, the sinus venosus is really formed by a fusion 
of these vessels from each side. 

Ductus Cuvieri or Cuvierian Sinuses. These are a pair of large veins 
which enter into the sinus venosus also and may even form part of that 
organ. They come from the body-wall opposite the sinus venosus. 

Hepatic Vein. As the liver develops, the omphalomesenteric veins, 
which pass through that organ, break up into capillaries within the sub- 
stance of the liver. Then the parts of both omphalomesenteric veins, 
which lie between the liver and the heart, fuse into a single hepatic vein. 

Hepatic Portal Vein. The right omphalomesenteric vein disappears 
caudad to the liver, while the left partly remains as the root of the future 
hepatic portal vein. This vein will later receive branches from the 
digestive tract as well as from those organs which have arisen from the 
digestive tract. 

Anterior Cardinal Veins. As the ducts of Cuvier pass dorsad to the 
dorsal body wall, they divide. One branch passes headward as the 
anterior cardinal vein. 

Superior Jugular Veins. This is the name given the anterior 
cardinal veins as they pass forward into the head, where they drain the 
brain and the dorsal portions of the head. 

Inferior Jugular Veins. These drain the mouth, sucker, and ventral 
surface of the head, and open into the roots of the duct of Cuvier just 
before these in turn enter the sinus venosus. 

Posterior Cardinal Veins. These are the posterior or caudal 
portions of the divided ducts of Cuvier, and are primarily the drainage 
system of the body-wall and excretory system. They pass caudad along 
the medial side of the pronephric ducts and receive the veins from the 
body-wall known as the segmental veins. The posterior cardinal veins 
form large sinusoids in the region of the pronephros, but as the meta- 
nephros develops, all this is modified, so that at fifteen millimeters the 
caudal ends of the veins fuse to form the single median cardinal vein. 

Caudal Vein. This begins at the tip of the tail and drains that 
region. It is unpaired, but upon reaching the body cavity, divides above 
the cloacal region, and then empties into the posterior cardinal veins. 

Posterior or Inferior Vena Cava, or Postcaval Vein. This begins 
as a branching of the left omphalomesenteric vein lying dorsal to the 
liver. From here, the postcaval vein passes through the suspensory fold 
of the liver to the right posterior cardinal vein and connects with it just 
anterior to the point where the median cardinal vein begins. This vessel 
enlarges rapidly to become the largest blood vessel in the body. It 
passes through the liver to the sinus venosus. The hepatic vein then 
opens into it instead of into the sinus venosus as formerly. 

Anterior, Superior, or Precaval Veins. The pronephric portions of 

The Circulatory System 177 

the postcaval veins degenerate as the pronephroi degenerate, and 
ultimately disappear entirely, even before metamorphosis is complete. 
These leave the ductus Cuvieri as the proximal portions of the anterior 
cardinal veins, and it is these remaining proximal portions which are 
called the anterior, superior, or precaval veins. All blood from the 
posterior parts of the body-wall and from the tail now passes directly to 
the heart through the median cardinal and postcaval veins. 

Iliac Veins. The pronephroi are followed by the mesonephroi as 
in the chick, and an alteration in the relation of the median cardinal vein 
follows. On each side of the body the developing mesonephroi push 
into the median cardinal vein, so that this vein is divided into one 
median and two lateral parallel channels. The caudal vein empties into 
the median channel and finally disappears, and the iliac veins, which 
come from the hind-legs, open into the lateral channels. It is the iliac 
veins which become the chief vessels leading to the mesonephric region 
after the caudal vein disappears. 

Adult Venous System. After an understanding of the formation 
and change which takes place in the venous system during the embryonic 
period, the adult system can be understood. 

Afferent or Advehent Mesonephric Veins, or Renal Portal Veins. 
These are merely the iliac veins, together with the lateral channels of 
the median cardinal vein, with which they are continuous. 

Posterior Vertebral Veins. These are the small veins from the 
posterior body wall which open mto the renal portal vein. 

Renal Veins, or Revehent Mesonephric Veins. These are the short 
connecting vessels which connect the vascular space in the mesonephroi 
with the median channel of the median cardinal vein, so that only this 
median channel remains as a posterior continuation of the postcaval 

Lateral Veins. A pair of these develop late in the ventral abdominal 
walls, and open into the sinus venosus. The lateral veins connect with 
the iliac veins posteriorly, then fuse medially. 

Anterior Abdominal Vein. The anterior ends of the lateral veins 
lose their connection with the sinus venosus, while the anterior portion 
of the right lateral disappears entirely. The left lateral vein forms a 
new connection with the hepatic portal vein, and is then called the 
anterior abdominal vein. 

Pulmonary Veins. These can be seen, when the tadpole is about 
six millimeters in length, as projections of the endothelium on the dorsal 
side of the sinus venosus. These projections form a tube, opening 
proximally into the left side of the auricle, which distally leaves the wall 
of the sinus venosus, and passes dorsally to the lung rudiments. This 
tube bifurcates at the base of the lungs, Avhere each branch then passes 
along the medio-ventral side of the lung rudiment. After the lungs begin 
to function, the pulmonary veins empty into the left auricle. 


The Embryology of the Frog 


By the time the tadpole is 6.5 milUmeters in length, one may see 
a single pair of "lymph hearts" (Fig. 11, Vol. I). They are sac-like, and 
grow out of the intersegmental veins, usually from the fourth pair. That 
is, they are outgrowths from the veins which run between the fourth and 
fifth myotomes. These "lymph hearts" empty into the posterior cardinal 
veins at the more caudal end of the pronephros. 

The "hearts" themselves lie between the peritoneum and the outer 
covering, and below the level of the m3^otomes. The endothelial lining 
of the "hearts" and the blood vessels is continuous. 

The "beating" of the lymph hearts is due to a syncytial layer or net- 
work of striated muscle fibers immediately outside of the endothelium. 
The "beating" begiA,| about the time the mouth opens. 

A short time after hatching, that is, when the tadpole is about 7.5 
to 8.0 millimeters in length, two lymphatic vessels develop from each 
heart. They are known as anterior and posterior lymph vessels. They 
follow the lateral nerve in direction, the anterior vessel extending into 
the head, and the posterior along the sides of the trunk. Valves guard 
the openings of the lymph vessels into the "hearts" as well as into the 
veins where they empty. 

Fig. 347. 

Frog. A, showing anterior lymph-hearts, from the dor- 
sal side. B, showing posterior pair of lymph-hearts seen 
from the ventral side, gl, gluteus muscle; ic, iliococcygeal 
muscle; L, lymph-heart; Is, levator scapulae muscle; A'', spinal 
nerve; p, piriformis muscle; r, vastus muscle; ts, transverse 
scapularis major muscle; ve, vastus externus muscle; 1-5, 
vertebrae. (After Wiedersheim.) 

Immediately after these lymph vessels begin growing, they develop 
a rich network of capillaries which spread out in all directions. They 
are greatest in number close to the skin. Later, as the tadpole becomes 
quite large (about twenty-six millimeters), the lymphatic system 
becomes well developed. 

The anterior lymph vessel, running downward and forward, connects 
with a large lymph sinus, around the mouth, heart, and branchial region. 
The posterior vessel passes caudad into the tail, and there divides into 
dorsal and ventral branches. These dorsal and ventral branches of each 

The Circulatory System 


side then unite to form two large vessels which extend through the tail, 
one of them above and the other below the myotomes. 

The walls disappear in the network that has grown out from the 
lymphatic vessels to form the large subcutaneous lymph sacs already 
noticed in the dissection of the adult frog. 

The thoracic ducts extend posteriorly from the "lymph hearts" and 
are probably outgrowths from them. They lie between the dorsal aorta 
and the posterior cardinal veins. 

The posterior lymph hearts (Fig. 347) — (one to three pairs in 
number) — develop from the intersegmental vein just as did the anterior 
hearts, but their development is postponed until the hind-legs appear. 

1 ; / 

Fig. 348. 

Diagrams to illustrate the divisions of the coelom in the various vertebrate 
classes. The transverse septum and its derivatives are indicated by thick lines. 
A, fishes, showing the division of the coelom into pericardial cavity a, and pleuro- 
peritoneal cavity g, by means of the transverse septum d. B., urodeles; similar 
to fishes with the addition of the lung h which projects into the pleuroperitoneal 
cavity ff, C, turtle; the pericardial cavity a has descended posteriorly until it 
lies ventral to the anterior part of the pleuroperitoneal cavity g; the anterior face 
of the transverse septum, d, has now become part of the wall of the pericardial 
sac; the lung, h, is retroperitoneal. D, early stage of Mammals, showing the 
beginning of the coelomic fold (pleuroperitoneal membrane), j, descending from 
the dorsal body-wall, and the liver, /, enclosed within the transverse septum, d, 
E, later stage of mammals, showing union of the coelomic fold, j, with the 
transverse septum d, the two together forming the diaphragm which separates the 
pleural cavity k from the peritoneal cavity, m; the liver has constricted from the 
main part of the transverse septum, the constriction becoming the coronary liga- 
ment, i, a, pericardial cavity; b, heart; c, parietal pericardium or pericardial sac; 
d, transverse septum; e, serosa of the liver, this being a part of the transverse 
septum originally; /, liver; g, pleuroperitoneal cavity; h, lung; i, coronary liga- 
ment of the liver; /, coelomic fold which forms part of the diaphragm; k, pleural 
cavity; /, pleuropericardial membrane or anterior continuation of the transverse 
septum; m, peritoneal cavity. (From Hyman's "A Laboratory Manual for Com- 
parative Vertebrate Anatomy," by permission of the Chicago University Press.) 

180 The Embryology of the Frog 

Their first openings are into the posterior cardinal vein, which means 
that later they empty into the renal portal veins. 

The Spleen. This ductless gland is first seen in the developing 
tadpole at about ten millimeters. It appears as a mass of mesenchymal 
lymphoid cells in the mesentery, immediately dorsal and posterior to the 
stomach and around the mesenteric artery. 

These cells then multiply and project from the mesentery so as to 
have a peritoneal covering, as do all the organs in the body cavity. 
Later, as the spleen enlarges, various wandering cells from the intestinal 
epithelium seem to be added to it. The spleen is complete and extremely 
vascular by the time the tadpole is twenty-five to twenty-seven 
millimeters in length. 


The pericardial cavity, already discussed, has remained open 
posteriorly into the abdominal cavity, with the exception of the region 
covered by the liver. Now, as the ducts of Cuvier form and pass from 
the body-wall to the sinus venosus, they pass through this open region 
and carry with them incomplete peritoneal folds from the body-wall. 
These folds are called the lateral mesocardia. They remain incomplete 
dorsally for a long time, but gradually extend ventrally so as to form a 
complete separation between pericardial and peritoneal cavities. This 
transverse partition is called the pericardio-peritoneal septum or the 
septum transversum. To this septum transversum is added a medial 
portion of peritoneum from the anterior face of the liver, while on the 
right side the septum becomes continuous with the posterior suspensory 
fold of the liver commonly called the mesohepaticum. 

After metamorphosis, the septum unites dorsally with the dorsal 
mesentery and completes the separation between pericardial and 
peritoneal cavities. 



FROM our study of the urogenital system of the chick, we learned 
that while in birds and mammals, a pronephros, a mesonephros, and 
a metanephros form, in amphibians, the first two forms of nephridic 
organs alone make their appearance, the mesonephros then remaining as 
the permanent adult functioning kidney. 

We have already spoken of the nephrotomes, which are also called 
the intermediate cell mass (Fig. 268). 

Some time before hatching, when the embryo is only about three 
to four millimeters in length, this intermediate cell mass can be seen as 
longitudinal thickenings on each side of the notochord. These thicken- 
ings then form a groove, the lips of which soon fuse to form a tube or 
duct. This is the pronephric or segmental duct. It is at the anterior 
end of this pronephric duct (in the region of the second to fourth 
somites) that the pronephros or head-kidney forms as a ventro-lateral 

At this anterior end of the pronephric duct three tiny openings are 
left as the lips of the duct fuse. These three openings become the three 
pronephric tubules (Figs. 349, 350),' and the openings of these tubules 
into the coelom are called the nephrostomes. 

The nephrostomes become lined with large cilia which produce a 
current out of the coelom. This current then passes by way of the 
pronephric duct to the cloaca. 

The pronephros itself becomes quite vascular. In the discussion 
of the posterior cardinal veins, mention was made of the close relation 
of these veins to the excretory system. It will be remembered that they 
lie along the pronephric ducts. The elongating of the phronephric 
tubules pushes them upward into the posterior cardinal sinus until the 
sinus is nearly filled. This means, of course, that the tubules are really 
bathed in venous blood. At the same time that this occurs, arterial 
blood is brought from the dorsal aorta to the excretory system by arteries 
in the form of glomeruli. 

The manner in which the glomeruli form is rather complicated. 
Opposite the second nephrostome, a fold appears in the splanchnic 
mesoderm, when the embryo is about 4.5 millimeters long. This fold 
lies parallel to the pronephros itself, becoming- elevated and projecting 
into the coelom opposite the nephrostomes. Vascular spaces appear in 
this fold, which develop into long convoluted vessels of the glomerulus 
proper, and also into a definite vessel which connects with a branch from 
the dorsal aorta. 


The Embryology of the Frog 

This region of the body cavity is later cut off from the pronephric 
chamber by the lungs, projecting laterally, and carrying a fold of 
peritoneum across to the peritoneum which covers the pronephros, fusing 
with it for a short distance. The pronephric chamber remains open into 
the coelom both anteriorly and posteriorly to the lung region. 

The pronephric capsule is derived from two sources, namely : from 
the ventro-lateral walls of the myotomes, which normally give rise to 
mesenchyme but which here evaginate in the pronephric region over 





Fig. 349. 

A and B. Diagrams of the development of the excretory system of the frog. 
A, The system of a tadpole about 12 mm. long, showing the pronephros and origin 
of the mesonephric tubules; B, the system at the end of the metamorphosis. The 
broken line represents approximately the position of the strip of peritoneal epithelium 
which gives rise to the oviduct. cL, Cloaca;, dorsal aorta; f.b., fat body; 
gl., glomerulus; gr., genital ridge; mes., mesonephros; ms.t., mesonephric tubules; 
od., oviduct; ovf., position of oviducal opening; pn.f., pronephric funnels; pnp., 
pronephros; sq., segmental duct. (From Bourne.) 

C, Diagram to show the structure of the pronephros and the mesonephros. 
Pronephros on the right and mesonephros on the left. The chief difference is in 
the relation of the _ glomerulus ; in the pronephros it projects into the coelom; 
in the mesonephros it projects into the tubule, which forms the Bowman's capsule 
about it. (From Wiedersheim.) 

The Urogenital System 


both dorsal and lateral surfaces of the head-kidney to meet with folds 
coming up from the somatic layer of the lateral plate. This forms a 
capsule of connective tissue which encloses not only the pronephros 
proper, but also the pronephric sinus of the posterior cardinal vein. 

The pronephros is largest when the tadpole is about twelve milli- 
meters long. It remains stationary until the twenty millimeter stage, 
when it begins to degenerate. Degeneration is not quite complete at 
metamorphosis. Various blind outgrowths of the three original tubules 
can be seen before degeneration sets in. The pronephric duct closes 
just posterior to the pronephros and the tubules break up and disappear. 

\ \ 

• <iiB|i)ao(m||j 


Fig. 350. 

Diagrams to show the development of the three kidneys and their ducts and 
their relation to the male gonad. A, early stage showing the pronephros a, de- 
veloping from the anterior end of the mesomere c and the pronephric duct b, which 
has not yet reached the cloaca e, B, next stage illustrating the degieneration of 
the pronephros at /, the development of the mesonephros h, from the middle portion 
of the mesomere, the junction of the pronephric duct, now the mesonephric duct 
g with the cloaca and the beginning of the metanephric evagination t from the 
mesonephric duct. C, later stage, showing connection between certain tubules of 
the mesonephros and the testis / by means of tubules, the vasa efferentia, p, 
which grow out from the mesonephros; and the penetration of the metanephric 
evagination into the posterior end of the mesomere where it is subdividing to 
form the _ collecting apparatus /, which becomes associated with the secretory 
metanephric tubules m, developed in the mesomere. D, final stage, in which the 
mesonephros has disappeared except for the remnant q, which connects with the 
testis ;■ by means of the vasa efferentia p; the mesonephric duct g persists as tlie 
vas deferens; the two parts of the metanephros shown in C have united to form 
a single organ r. a, pronephros; h, pronephric duct; c, mesomere or nephrotome; 
d, intestine; e, cloaca; /, degenerating pronephros; g, mesonephric or Wolffian 
duct; h, mesonephros or Wolffian body; i, metanephric evagination from the Wolffian 
duct in B, ureter in C and D; j, testis; k, coiled portion of the vas deferens forming 
part of the epididymis; /, collecting part of the metanephros derived from the 
Wolffian duct; m, excretory tubules of the metanephros derived from the mesomere; 
n, nephrostome; o, renal corpuscle or Malpighian body; p, vasa efferentia; q, remnant 
of the mesonephros, forming part of the epididymis; r, metanephros. (From Hyman's 
"A Laboratory Manual for Comparative Vertebrate Anatomy," by permission of 
The Chicago University Press.) 

184 The Embryology of the Frog 

The nephrostomes, hoAvever, approach one another, and finally meet 
to open into a common cavity, called the common nephrostome. This, 
then, also closes so that the nephrostomes are entirely cut off from all 
communication with the body cavity. The glomeruli also disappear, 
although traces of these can still be seen for some months after 


Just about the time the pronephros attains its full size, or a little 
before, the mesonephros, or Wolffian body, begins to form in the region 
of the nephrotomes or intermediate cell-mass of the seventh to twelfth 
somites. It is both somatic and splanchnic in origin. These nephro- 
tomes fuse in a continuous longitudinal strip of irregularly arranged 
cells which lie between the pronephric duct and the dorsal aorta, along 
the posterior cardinal vein. Little swellings appear in this mass, which 
are the beginnings of the mesonephric vesicles. They are more numerous 
than the mesodermal segments, and so are not strictly metameric. 

Each of these swellings divides into a large ventral and a smaller 
dorsal chamber; the larger one is called the primary mesonephric unit 
and the smaller the secondary mesonephric imit. 

The secondary units divide later to form a tertiary mesonephric 

Each of the three units develops much alike. Figures 349 and 350 
show this development. There are two outgrowths, an inner tubule 
Avhich grows dorso-laterally to the pronephric duct where it opens, and 
an outer tubule which grows ventro-medially to the peritoneum with 
which it fuses, and then it empties into the body cavity. 

The inner tubules connecting with the mesonephric duct convert 
the portion where such connections are formed into the mesonephros or 
Wolffian body — the true kidney of the adult frog. The duct itself, in 
the region of the connections, is known as the Mesonephric, or Wolffian, 
Duct, while the duct posterior to this region is now the ureter. 

The inner tubules become elongated and coiled to form the tubular 
portion of the mesonephros, while the outer tubules form outgrowths 
which later form the capsule around the glomeruli, known as Bowman's 

A small twig from the dorsal aorta connects with each glomerulus 
in a similar manner to the way the glomeruli were formed in the 
pronephros. The proximal portion of the outer tubule, from which the 
outgrowths arise to form the capsule, now separates from the remaining 
tubule, but retains its connection with the inner tubule, and the distal 
portion of the outer tubule comes to lie in connection with the body 
cavity. This latter connection becomes ciliated and forms a typical 

The Urogenital System 185 

nephrostome as in the pronephros. The nephrostomal region now forms 
another connection at its inner end with the sinus of the posterior 
cardinal vein, in which sinus the mesonephric tubules lie surrounded by 
venous blood. 

Many, as high as two hundred, outer tubules and nephrostomes may 
be formed from the three units described, and possibly some may be 
formed also by independent evaginations from the peritoneum, or even 
by splitting of those previously formed. 

The urinary bladder is a median ventral outgrowth from the wall 
of the cloaca nearly opposite the openings of the ureters. 


The mesonephric duct becomes divided somewhat obliquely into 
two portions in front of the mesonephros, the more anterior portion now 
being the Miillerian duct, while the posterior portion forms the Wolffian 
duct. The Miillerian duct connects with the peritoneal epithelium 
anteriorly and empties posteriorly into the cloaca. In the male frog, this 
duct persists as a mere longitudinal streak on the outer side of the 
kidney, and extends some distance in front of it. In the female frog, this 
Miillerian duct becomes the oviduct (Fig. 350). 

The Wolffian duct functions as the ureter in both sexes, but in the 
male, the posterior end of it becomes dilated into a glandular enlarge- 
ment, called the seminal vesicle. 

Already at the six millimeter stage, as the mesentery is being formed 
by the coming together of the somites from both sides just under the 
dorsal aorta, a small group of entodermal cells is pinched off from the 
yolk, which, after completely separating from the yolk, divide longi- 
tudinally, each half moving laterally. These longitudinal halves are the 
genital ridges (Fig. 349, B). They lie close to the mesenteric attach- 
ment, and just beneath the cardinal veins. 

The genital ridges become quite conspicuous in a short time by 
germ-cell proliferations as well as by the peritoneal cells which cover 
them and the mesenchymal cells from the body wall which migrate to 
this region. 

The mesenchymal cells form the stroma of the ridge; the peritoneal 
cells form a thin superficial covering at first, while later they also form 
the suspending folds (mesorchia of the testes, and mesovaria of the 
ovaries) of the gonads. The germ-cells now begin to proliferate and 
form the nests of cells (Fig. 254) which are to develop into gonads and 
gametes as already described early in the embryology of the chick. 

The anterior portion of the genital ridge becomes the fat body 
shortly before metamorphosis, while the posterior portion connects with 
the mesonephric duct. 

In this posterior region several outgrowths from the Malpighian 


The Embryology of the Frog 

bodies, known as sexual cords, can be seen. These become tubular, and 
extend into the substance of the gonad. In the male, these sexual cords, 
after metamorphosis, form the vasa efferentia, or efferent ducts, by which 
spermatozoa are carried from the gonad proper to the real sperm duct, 
the vas deferens. In the female, the portions between ovary and meso- 
nephros degenerate, remaining only as a vestige, called Bidder's organ 
(Fig. 457). 

The tadpole must be of considerable size before the sexes can be 
distinguished. Bouin gives the length as thirty millimeters in Rana 
temporaria. In the male gonad, the cells all look alike, while in the 
female gonad, the follicle arrangement can be made out, and the ovary 
acquires a central lumen. 


Figure 351 will show how the adrenal bodies grow on the meso- 
nephros of the frog. The important point to remember is that there are 

Fig. 351. 

Parts of sections through young R. temporaria, show- 
ing the origin of the adrenal bodies. A. Through 30 mm. 
tadpole. B. Through 11 mm. frog after metamorphosis. 
a. Dorsal aorta; ac, cortical cells of adrenal body; antj 
medullary cells of adrenal body; ct, connective tissue; g, 
gonad; gs, sympathetic ganglion; m, mesentery; n, mesoneph- 
phros; rv, revehent renal vein; v, vena cava; x, point where 
ganglion cells enter mesonephros and adrenal body. (After 

two kinds of cellular substances in these organs. The adrenal bodies lie 
on the ventral surface of the mesonephros in the frog. 

Histologically, one may see a coarse network of cell strands with 
occasional groups of darkly staining tissue, called phaeochrome tissue. 
Blood from the median posterior cardinal vein occupies the spaces in 
the adrenal body. The coarse network forms what is called the cortical 
tissue, while the more darkly staining portions are known as medullary 
tissue, because in the higher forms of animal life, the darkly staining 
portion lies toward the inner region of the organ, and the coarse network 
lies toward the outer or cortical region. 

When the tadpole is about twelve millimeters in length, the cortical 

The Urogenital System 187 

region appears as small groups of cells lying along both sides of the 
wall of the median posterior vein, below the level of the mesonephros, 
as well as beneath the peritoneal epithelium from which they seem to 

Just after metamorphosis, these cell-groups separate from the 
peritoneum to form the network mentioned. 

The medullary portion, however, has a totally different origin, and 
one which may throw light on further work in the study of ductless 
glands, whose secretions have become an important factor in modern 
medicine. This portion is derived from the ganglia of the sympathetic 
nervous system by groups of cells whose precise origin is not clear. 
Some of these cell groups remain in the sympathetic ganglia, but others 
migrate to the adrenal body and become scattered about. 



THE notochord extends from the blastopore to the pituitary body, 
as a rod of vacuolated cells filled with fluid, around which three 
layers or sheaths form. 

The primary or elastic sheath, is an outgrowth of superficial cells 
of the notochord itself and forms the superficial surface sheath of the 

The secondary or fibrous sheath is formed between the primary 
sheath and the notochord also by cells from the notochord itself. 

The third or skeletogenous sheath forms on the outside of the 
primary layer at a later period as a thin sheath which is formed by the 
sclerotomes. The sclerotomes, it will be remembered, are outgrowths 
from the somites. This skeletogenous layer extends dorsally, entirely 
around the neural tube, and laterally, from the notochord, in between 
the successive myotomes. It is in this skeletogenous layer that the 
vertebrae are to be formed. 

When the tadpole is about fifteen millimeters long, a series of 
metameric cartilages can be observed along the medio-ventral surface of 
the notochord. They lie in the skeletogenous sheath (Fig. 352, cs). 
These segments fuse longitudinally to form a pair of dorsal and ventral 
strips which extend along the entire notochord. 

These strips now become metameric by constrictions of fibrous tissue 
which form rings. The rings are the beginnings of the inter-vertebral 
ligaments, which, just as in the chick, appear opposite the middle of each 
mesodermal segment. 

The mesodermal segments become the vertebrae, so that the liga- 
ments which form as separate segments between the vertebrae, are able 
to act on both the vertebrae lying immediately anterior and immediately 
posterior to each mesodermal segment, after the muscle has developed 
in connection with these ligaments (Figs. 305, 352). 

The notochord becomes segmented and surrounded by cartilage, the 
notochordal segments form the soft centrum of the vertebrae, and 
probably also portions of the intervertebral discs. 

The ventral cartilages now grow around the sides of the notochord 
and meet to fuse with the dorsal series. 

The transverse processes of the vertebrae grow out from the ventral 
cartilages. It is toward the lateral ends of these that the transverse 
processes of the ribs later develop. 

The neural arch is formed from outgrowths of the dorsal series 
which grow inward beneath the neural cord and also dorsally and ven- 

The Skeletal System 


trally. Later, when ossification begins, short processes called inter- 
vertebral articulatory processes develop from the neural arches, by 
which each vertebra joins with the next succeeding vertebra. 

Ossification begins in the tadpole between the dorsal and ventral 
series of cartilages, just described. There are nine vertebrae formed in 
the frog, plus the urostyle, the latter being unsegmented. 

Cross section through a developing vertebra, rib and 
exoskeleton of a Turtle, c, corium in which the dermal plates 
are developed; cs, primitive vertebral body; ep, epidermis; 
m, external oblique muscle; p, perichondrium; r, rib; sp, 
spinous process. (From Kingsley after Gotte.) 


The skull is commonly formed from various embryological elements, 
which may be listed as follows : 

(1) Cranium. 

(2) Sense Capsules. 

(3) Visceral Arches. 

(4) Notochord. 

(5) Vertebrae. 

(6) Membrane or Derm Bones. 

It will be remembered that there are no true segments in the head 
region of the frog. Consequently, the list just given is only assumed 
from a comparison of other forms. 

When the cranial region begins its cartilage formation, the tadpole 
is about seven millimeters in length. A pair of dense strands of tissue 
form along the ventro-lateral surfaces of the fore-brain. These then 
become cartilaginous and form the beginnings of the trabeculae or 
trabecular cartilages (Fig. 310). These trabeculae extend forward and 
fuse across the midline between the olfactory organs. The fusion forms 
the internasal plate. The trabeculae continue extending forward, and 
these extensions are known as the trabecular cornua, at the ends of 
which the olfactory capsules form. 

A pair of labial or supraostral cartilages, which have formed in the 
upper lip, meet with the olfactory capsules. 

The notochord, which extends into the brain up to the pituitary 
body, has a pair of tissue thickenings beginning in the region of the 


The Embryology of the Frog 

hind-brain and extending anteriorly as parachordae, or parachordal 
cartilages. These parachordal cartilages now fuse with the posterior 
ends of the trabeculae to enclose the tip of the notochord, and the entire 
continuous plate beneath the fore-brain is then called the parachordal 

These parts can be made understandable only by a careful examina- 
tion of Figures 310 and 353, which must be studied with great thorough- 
ness or much of the later work in comparative anatomy will be valueless. 

From the visceral arches, the palato-quadrates are formed as a pair 
of flattened rods, lateral to the trabeculae. These are in intimate rela- 

Fig. 353. 

A, Chondrocranium of 29 mm. larva of R. fusca. To the left, the ventral 
surface; to the right, the dorsal surface, a, Auditorj' capsule; bp, basal plate; c, 
notochord; ct, trabecular cornu; /, basicranial fontanelle; fa, foramen for carotid 
artery; fm, foramen magnum; fo, foramen for olfactory nerve; ir, infrarostral 
cartilage; /, jugular foramen for IX and X cranial nerves; /, perilymphatic 
foramina; m, muscular process; M, Meckel's cartilage; o, otic process of palato- 
quadrate; pf, palatine foramen; pg, palato-quadrate cartilage; sr, suprarostral 
cartilage; t, trabecular cartilage; v, secondary fenestra vestibuli. B, Anterior portion 
of chondrocranium of R. fusca during metamorphosis. Lateral view. C. Skull 
of 2 cm. R. fusca, after metamorphosis. Dorsal view. Membrane bones removed 
from left side, a. Auditory capsule; am, anterior maxillary process; an, annulus 
tympanicus; art, articular process of palato-quadrate cartilage; eo., exoccipital bone; 
/, fronto-parietal bone; fpo, prootic foramen; mx, maxillary bone; n, nasal bone; 
o, olfactory cartilages; on, orbito-nasal foramen; pa, anterior ascending process of 
palato-quadrate; pg, pterygoid bone; pi, plectrum; pm, posterior maxillary process; 
pp, posterior ascending process of palato-quadrate; pq., palato-quadrate cartilage; 
pt, pterygoid process of palato-quadrate; px, premaxillary bone; qj, quadrato-jugal 
bone; //, foramen for optic nerve; ///, foramen for /// cranial nerve; IV, foramen 
for IV cranial nerve. (From Ziegler.) 

tion to the cranium proper. They connect with the trabeculae by 
anterior ascending processes back of the olfactory region, and by pos- 
terior ascending processes opposite the end of the notochord. 

The remaining portion of the skull, which develops from the visceral 
arches, is connected with the jaw, and will be described shortly. 

The infundibulum and pituitary body lies within the basi-cranial 
fontanelle, which is the open space just anterior to the tip of the noto- 
chord. From now on, development continues mostly in the posterior 
portion of the cranium. 

Figures 310 and 353 show how the auditory organ is formed by a 
connective tissue capsule which soon becomes cartilage, while the 
mesotic cartilage grows out posteriorly and laterally from the para- 

The Skeletal System 191 

chordal plate to unite with the auditory capsule ventrally, both anteriorly 
and posteriorly. 

The occipital cartilage is a continuation of the mesotic cartilage 
which fuses with the auditory capsule, and leaves a small opening 
through which the IX and X cranial nerve pass. This opening is called 
the jugular foramen. 

The basal plate is the name given to the floor of the posterior 
portion of the cranium, which consists of occipital and mesotic cartilages 
together with the parachordal plate. 

The occipital cartilage extends dorsally around the neural cord to 
form the foramen magnum. 

The auditory capsule remains open into the cranial cavity internally 
by a large foramen, but closes externally. 

The trabeculae now grow across the basicranial fontanelle so that 
it becomes entirely closed. This closed portion is the floor of the cranial 
cavity. The trabeculae then extend laterally and form the lateral walls 
of the cranial cavity, thus separating the cavity from the orbits. 

Cartilages from the trabeculae also extend dorsally across the mid- 
line in the anterior region, to form a narrow dorsal bridge, leaving a 
large supracranial fontanelle between this bridge and the supraoccipital 

The internasal septum extends dorsally and becomes the anterior 
wall of the cranial cavity, while the trabecular cornua remain separate 
from the olfactory capsules, but connect anteriorly with the supraostral 
or labial cartilages. During metamorphosis, however, both labial 
cartilages and anterior ends of the cornua disappear in front of the 
olfactory capsules. 

True bones form late in the frog. The following bones are the more 
important which have developed from cartilage: 

Exoccipitals, or Lateral Occipitals. These form from the posterior 
portions of the occipital cartilage and auditory capsule. The occipital 
condyles themselves as well as the median dorsal and ventral portions 
of the occipital region remain as cartilage. 

Pro-otics. These form from the more anterior portion of the audi- 
tory capsules as well as from the basal plate and orbital region. 

Ethmoids. These form in the anterior portion of the wall of the 
orbit. They then unite both above and below so as to from a band 
around the cranium, often also called the sphenethmoid or orbito- 

Quadrato-jugal. The palato-quadrate cartilage forms bone only in 
the region of the lower jaw. Then a connection is formed with a mem- 
brane bone and these two together form the quadrato-jugal. All these 
bones form before metamorphosis, the ethmoids alone developing some 
weeks after metamorphosis has taken place. 

The Visceral Skeleton. In the mandibular and hyoid arches, as well 

192 The Embryology of the Frog 

as the three branchial arches, the various skeletal elements appear as 
condensations in the mesenchyme, which soon become cartilaginous. 

First, a short rod appears in the mandibular arch, transverse to the 
axis of the embryo. 

This divides the dorsal portion into the beginnings of the upper jaw 
or palato-quadrate, and the ventral portion which is the beginning of 
the lower jaw. The lower jaw elements become subdivided into 
Meckel's cartilage, which comes to form the true jaw, and the infra- 
rostral cartilage. 

The palato-quadrate has grown rapidly, as already described, and 
then fused with the trabeculae. 

When the tadpole is about twenty-one millimeters long, the pos- 
terior or quadrate portion of this same cartilage connects with the 
auditory capsule. 

With metamorphosis, the mouth enlarges, and this pushes back 
many of these structures, while the part of the palato-quadrate which 
lies in the orbital region, softens and disappears to a considerable extent. 
The anterior connection of palato-quadrate and trabeculae becomes the 
future pterygoid and palatine regions. All these changes draw the jaw 
to the posterior portion of the cranium from its original anterior position. 

The infra-rostral cartilages, which have fused together across the 
midline, now fuse with the Meckelian cartilages to form the apex or 
mental portion of the chin. The fused cartilages are now known as 
mento-Meckelian cartilages. As these ossify, they fuse with the dentary, 
which is really the chief membrane bone of the lower jaw. There is a 
small median element between the infra-rostrals which also fuses with 

The annulus tympanicus is the outgrowth from the quadrate car- 
tilage which surrounds the tympanic membrane of the frog. It does not 
complete its growth until long after metamorphosis. 

The hyoid arch, like the three branchial arches lying posterior to it, 
makes its appearance as a pair of rods of dense tissue in the correspond- 
ing visceral arches, though not at the same time as the others. 

The hyoid cartilage, also called the ceratohyal cartilage, extends 
dorsad and connects with the palato-quadrate immediately behind where 
the jaw articulates. Ventrally, it unites with the hyoid cartilage of the 
opposite side. (Fig. 354.) 

The first branchial cartilage also unites in the ventral midline, while 
the remaining branchial arches do not unite in the midline ventrally, 
but have their lower anterior ends unite with the one lying immediately 
anterior to it, and, finally, they connect dorsally in a similar manner. 

The copula, which is a medial element, then appears in the ventral 

The Skeletal System 193 

region of the pharynx between the hyoid and the first branchial, and 
connects the ventral ends of both these arches. 

The hypo-branchial plate consists of the lower ends of the first 
branchials which have become flattened and expanded. The ventral 
ends of the other three branchials fuse with the hypo-branchial plate. 

6A The cerato-branchials are the lateral and mid- 

dle sections of the branchial cartilages between the 
visceral pouches which remain separate from one 

At metamorphosis, when the gill slits close, 
many changes naturally must take place in the 
Fig. 354. _ structures just described. For example, the hyoid 
archer of a^29 mm^ larva bar loscs its conucction with the palato-quadratc, 
vLw.^' bb!*"^^BasibranchSi and bccomcs Smaller in diameter, while the copula 
Ss7h%;°'c/i/°S'ra^tohyat; Hkcwise bccomcs Smaller and a pair of new car- 
1% ^ivlt'^to'^iour cSaS tilagcs dcvclop on each side of it, which then con- 
branchiais. (From zieg- ncct the hypo-brauchial plate with the hyoid por- 
tions. These are the manubrial cartilages. 

The hyo-branchial apparatus of an adult frog is made up of a broad 
median plate of cartilage which has been formed by the fusion of 
manubrium, copula, and hypo-branchial plate. The hyoid cartilages 
remain as slender processes called the hyoid Gornua. The remaining 
portions practically disappear. 

The membrane bones. In those portions of the cranium where con- 
siderable stretching has taken place, such as in the roof of the skull and 
the lining of the mouth, the substance is thinner than in the cartilaginous 
portions, and is then called membrane. 

Membrane is nothing more than stretched-out-cartilage. 

The Parasphenoid. This is a single median bone, and the first of 
all bones of the skull to appear, whether cartilaginous or membranous. 
It forms in the roof of the mouth when the tadpole is about twenty 
millimeters long. The parasphenoid becomes dagger-shaped and covers 
the entire basicranial fontanelle. 

The frontals and parietals, which are paired, appear later and cover 
the supracranial fontanelle. They later fuse to form the fronto-parietals. 

The nasals form the roof of the olfactory capsules and the septo- 
nasals or intra-nasals appear within the capsules. 

The premaxillae and maxillae are the membranous parts which be- 
come the margins of the upper jaw. 

The dentary and angular cartilages surround Meckel's cartilage ; the 
dentary connects with the infra-rostrals of Meckel's cartilage. 

The vomers are paired, and appear beneath the olfactory capsules. 

The palatines form across the anterior margins of the orbits. 

The pterygoids form along the inner faces of the palato-quadrate 

j^g^ The Embryology of the Frog 

The squamosals form along the outer face of the palato-quadrate 

guished from the cartilaginous bones of the skull. 



IN both the chick and the frog — the two forms we have thus far dis- 
cussed — the eggs have passed out of the body of the mother. In the 
frog, the entire embryo developed after the egg left the mother. In 
the chick where fertilization is internal, development began before the 
shell was formed, so that an embryo, approximately twenty-four hours 
old, was already present when the tgg was laid. 

Now we shall deal with viviparous animals, that is, with those which 
give birth to living young. 

It will be understood quite readily that in those cases where living 
young are brought forth, the development must take place within the 
body of the mother, but, even in viviparous animals there are sub- 
divisions. One subdivision is made up of such animals as the duckbill, 
the Australian ant-eater, the Australian kangaroo, and the American 
opossum. In these animals, known as Marsupials, the female bears a 
pouch in the abdominal region in which the young are placed at a very 
premature age. In fact, in the opossum, the embryo may be only about 
eight days of age when it is born for the first time, so to speak. The 
mother then places it within the pouch or marsupium, and here the 
young continue their development until able to lead an independent 

In all the higher forms of mammals with which the student is 
familiar, fertilization is internal, as in the chick, and the embryo passes 
through a process similar to that of the chick, except that this embryonic 
process takes place within the mother's body. 

There is, however, in viviparous animals no real yolk-supply as in 
both the chick and the frog egg. Consequently, there must be some 
kind of an arrangement by which the young not only become attached 
to the uterus of the mother, but there must also be an arrangement by 
which a blood-supply can pass from mother to offspring, thus taking 
the place of the nourishment which the yolk furnishes in egg-laying 

The mammalian egg, not possessing a yolk, is very small. 

The original development of egg and sperm, however, is not very 
dissimilar to that already described for the chick. 

Before entering into the study of mammalian embryology proper, 
it is well, at this point, to understand the terminology usually applied to 
the life-history of a mammal. 

First, the period of gestation or true embryonic period. It is durino- 
this time that the embryo depends upon its connection with the mother's 


Mammalian Embryology 

uterus for nourishment. Gestation extends from the time of the fertili- 
zation of the tgg to the time of birth. 

Second, parturition, or the actual time of birth. The condition of 
the offspring at the time of parturition varies to a considerable extent. 
Some animals are born with the ability to walk and take reasonable 
care of themselves within a very short time after birth, while some are 
quite dependent upon their mother for a long time. 

Third, the period of adolescence, which is that period of life in the 
young devoted entirely to growth and development. It extends from 
birth to sexual maturity. 

Fourth, adult life, or the period of sexual maturity. During this 
period many physiological changes often take place in the individual, 
entirely aside from those of the reproductive system. 


As the egg is thrown out of the Graafian follicle (Fig. 355), it passes 
into the oviduct (Fallopian tube) and is carried by the cilia in the oviduct 
to the uterus. If fertilization takes place, the sperm, which finds its way 

Fig. 355. 
A Section of well-developed Graafian follicle from human embryo (von Herff ) ; 
the enclosed ovum contains two nuclei. B. Ovary with mature Graafian follicle about 
ready to burst (Ribemont-Dessaignes). C- Section of human ovary, showing mature 
Graafian follicle ready to rupture. ^ 

Kollmann's Atlas. 

Mammalian Embryology 


into the uterus, passes into the oviduct in an opposite direction from 
that which the egg takes. This causes a meeting of egg and sperm. The 
length of time it takes the egg to reach the uterus, after ovulation, varies 
in different species of animals. It may vary from a few hours to several 
weeks. It is, therefore, practically impossible to state the exact time 
when fertilization actually takes place. This is especially true in the 
human being; but, as soon as the sperm does meet the egg, and fertiliza- 
tion does take place, the embryo begins developing. Consequently, by 
the time the fertilized egg reaches the uterus, it has already passed 
through, or is just passing through, a stage that is even a little advanced 
beyond the gastrula stage. There are, in fact, several germ-layers 
already present at this time. 


As soon as fertilization takes place, the egg divides equally into 
two cells, these two into four, and so on in the usual way. However, 
very early, some of the cells divide more rapidly than others, so that 
there is an overgrowth of those which grow most rapidly. This gives 
rise to several terms. The more rapidly growing, or outer layer, is 
called the sub-zonal layer, while the central mass is called the inner cell 

The sub-zonal layer is only one cell in thickness, so that it is easily 
distinguished from the inner cell mass. Then, too, a cavity forms 
between the two layers. 

The entire structure is now called a morula (Fig. 356). As soon 

Fig. 356. 

Morula and early blastodermic vesicles of the rabbit. The zona radiata and 
albuminous layer are not shown. A. Section through morula stage, forty-seven hours 
after coitus. B. Section through very young vesicle, eighty hours after coitus. 
Taken from uterus; ordinarily the ova have not reached the uterus at this age. 
C. Section through more advanced vesicle, eighty-three hours after coitus. Taken 
from uterus, c, Cavity of blastodermic vesicle; i, inner cell mass; w, wall of blasto- 
dermic vesicle (subzonal layer, trophoblast). (From Assheton.) 

D. Section through the fully formed blastodermic vesicle of the rabbit, fcm, 
Granular cells of the inner cell mass; troph, trophoblast cells; zp, zona pellucida. 
(From Quain's Anatomy, after Van Beneden.) 

as the cavity has definitely formed between the inner cell mass and the 
sub-zonal layer, the morula is known as a blastodermic vesicle. This 
cavity contains a fluid which is supposed to represent the yolk-mass of 
the blastula and gastrula in the lower forms. 

198 Mammalian Embryology 

It will be noted from what has just been said that considerable 
development has already taken place by the time the fertilized egg 
reaches the uterus. Or, to repeat what was said above, it is as a blasto- 
dermic vesicle that the mammalian egg reaches the uterus after 

At this stage, two important points must be considered: 
First, the method of the formation of germ-layers, and 
Second, the method by which the blastodermic vesicle attaches itself 
to the uterus of the mother. 

Formation of the Germ-layers. 

The inner cell mass spreads out rapidly so as to form an inner lining 
to the sub-zonal layer. It is this inner lining which is the entoderm. 

The sub-zonal layer becomes the ectoderm. 

As there is a tremendous variation in the way germ-layers are 
formed in mammals, it may be well to think of the following example 
as a help in understanding some of these variations. 

Suppose a group of football players who had already played together 
in previous years were to come together again. Each would immedi- 
ately take his place without any preliminary instruction. So, we may 
think of the embryonic cells in the higher mammals taking a definite 
place and then developing from there on, rather than passing through 
all the stages of gastrula formation. This gastrula then actually indents 
to form two layers. That is, we may think of those embryonic cells 
which are to develop into ectoderm and mesoderm actually taking the 
proper position to develop into these structures without first becoming 
a single sheet and then indenting. 

This would mean that the undifferentiated cells, which are to become 
ectoderm, would arrange themselves on the outer portion, those which 
are to become entoderm would arrange themselves more inwardly, and 
those which are to become mesoderm would take their place between 
these two layers, and then all three could begin developing at about the 
same time and grow simultaneously. 

In the lower mammals, such as the cat, dog, rabbit, etc., this inner 
cell mass (entoderm) keeps pace with the sub-zonal layer, so that the 
original cavity, which has formed between the inner cell mass and the 
sub-zonal layer, is now surrounded by an inner layer of entoderm, while 
the outer layer still remains sub-zonal. In the higher forms, such as the 
primates, that is, in man and the higher apes, the inner cell mass does 
not grow as rapidly as the sub-zonal layer. There is, therefore, a second 
cavity formed within the inner cell mass of entodermal cells. 

It is the remaining- portion of the inner cell mass, after the entoderm 
has thus separated from it, which is the ectoderm. The sub-zonal layer 
is then called the trophoblast (Fig. 356, D). This trophoblast serves 
as the attachment of the blastodermic vesicle to the walls of the uterus. 

We see from what has been said that a true mammalian gastrula 

Mammalian Embryology 


(although formed in a different manner from either that of the chick or 
the frog) has been established with two definite cell or germ-layers. 



There are two general ways in which the blastoderm may become 
attached to the uterus. 

The trophoblast or sub-zonal layer may remain as an outer layer 
around the entire blastoderm, or the developing embryo within the inner 
cell mass may push through this outer layer and come to lie in close 
relationship to the uterine wall. 

At about the same time that attachment of blastoderm and uterine 
wall takes place, the amniotic cavity is formed (Fig. 357). The 

Diagrams of the relations of the cavities and layers in the rat, showing 
the "inversion" of the germ layers. Median sagittal sections. Embryo and 
amnion, black; ectodermal knob or "trager" in light tone; endoderm and 
mesoderm in darker tone. A. Early stage before the formation of the false 
amnionic cavity. B. Late stage showing false and true amnionic cavities 
and the interamnionic cavity, a, Amnion; ac, true amnionic cavity; c, chorion; 
E, embryo (anterior end; ea, endodermal rudiment of allantois) ; /, false 
amnionic cavity; i, interamnionic cavity; m, mesoderm; ma, mesoderm of allantois; 
n, endoderm; o, trophoblast (ectoderm); p, anterior intestinal portal; ra, rudiment 
of true amnionic cavity; rf, rudiment of false amnionic cavity; s, marginal 
sinus; t, "trager" (ectoderm); y, yolk-sac; ye, yolk-sac endoderm; x, amnionic 
folds. (After Salenka.) 

trophoblast remains as an outer covering in man, in many primates, and 
in such animals as the mouse, rat and guinea pig. When the trophoblast 
remains as the complete outer covering such a condition is known as 
entypy, and it is in animals in which this condition occurs, that a definite 
space is formed between the germ layers and the trophoblast. This 
cavity is known as the amniotic cavity. 

Sometimes the trophoblast thickens in this particular region and 

200 Mammalian Embryology 

a second or false amniotic cavity may develop. Figure 357 will make 
this clear. 

In those cases, however, where the embryo pushes through the 
trophoblast and comes to lie as a disc upon its surface, the amnion is 
formed quite as it is in the chick. 

The region in which the embryo develops is known as the embryonic 
shield. The primitive head-node lies practically in the middle of the 
embryonic shield. The primitive streak and the primitive grooves form 
quite as in the chick, and all structures lying anterior to the head-node 
lie in the head proper. 

A definite notochord also forms, and the neurenteric canal can be 
seen quite plainly at the posterior limits of the embryonic rudiment. 

Scarcely more than half a dozen human embryos have been seen 
prior to the time of the formation of the medullary plate. Then, too, 
none of these were of the same size, so we do not even have a basis for 
valid comparison, and consequently, we are unable to judge whether 
any of these were normal in size and form. 

Mesoderm is formed in the mammal as it is in the chick, each 
mesodermal somite dividing into a somatic and a splanchnic layer. A 
layer of entoderm joins with the splanchnic mesoderm to form the 
yolk-sac, although no yolk is present. The trophoblast joins with the 
somatic mesoderm to form the chorion. 

Here we may note that the term *'ovum" is used in mammalian 
development to designate any early stage in the embryo, even to the 
inclusion of the entire blastodermic vesicle. The term "embryo" in man 
is given the embryo only during the first two months of its existence; 
thereafter (that is, when the face and body are quite well formed) it is 
known as a ''foetus." 

The smallest human embryo yet seen was 1.54 mm. in length, while 
the entire blastoderm was about 1 cm. in diameter. 


There are three methods by which the blastoderm attaches itself to 
the walls of the uterus: 

First, by what is called central implantation. This occurs in the 
ungulates and carnivores as well as in the lower primates and in some 
rodents, such as the rabbit. In these the blastoderm becomes super- 
ficially attached to the uterine wall, and, consequently, projects freely 
into the lumen of the uterus. 

Second, eccentric implantation. This type is found in the mouse 
and in some insectivora. In these forms the blastodermic vesicle lies 
in a furrow or groove in the uterine wall. This groove is then closed up 
so that the vesicle comes to lie in the walls of the uterus. 

Third, interstitial implantation. In this type the blastodermic vesicle 
actually burrows its way into the mucous membrane lining of the uterus. 

Mammalian Embryology 201 

It is this third type which occurs in man and in some of the rodents, 
such as the guinea pig and the gopher. 

The trophoblast, in the region where it is to meet with the uterine 
wall, has become highly specialized physiologically in the eccentric and 
interstitial types of implantation. Its cells form a layer of considerable 
thickness, and it is then called a trophoderm (Fig. 358). These cells are 
supposed to dissolve, or digest, the uterine mucosa so as to permit a 
definite implantation and also, probably, to digest some of the mucosa 
as food for the growing embryo. 

The blastoderm attaches itself to the uterine wall between the two 
oviducts, and it is in the region of implantation that the maternal tissues 
come into contact with the embryo. We must, therefore, look for the 
beginnings of the placenta in this region. 

Fig. 358. 

A. Diagrammatic section of placenta. (After Strahl, Bonnet.) 

B. Section through an embryo of 1 mm. embedded in the uterine mucosa 
(semidiagrammatic after Peters). Am., amniotic cavity; h.c, blood-clot; b.s., body- 
stalk; ect., embryonic ectoderm; ent., entoderm; mes., mesoderm; m.v., maternal 
vessels; tr., trophoderm; u.e., uterine epithelium; u.g., uterine glands; y.s., yolk-sac. 

In fact, it is the trophoderm which later becomes vascularized from 
the mesoderm of the chorion or allantois, to act as the chief absorptive 
surface through which, and by which, material from the maternal tissues 
and blood is taken to the embryo. 


It will be remembered that in the chick embryo, the amnion has as 
one of its functions the protection of the embryo from drying and from 
becoming deformed by the outer shell pressing against it. The chick's 
yolk-sac contains a large quantity of food-substance which the develop- 
ing embryo uses, and the allantois serves as a respiratory and (partially) 
as an excretory organ. In the chick the serosa or chorion was of little 

In the mammal it is the amnion which is of secondary importance. 


Mammalian Embryology 

The yolk-sac has no yolk in it and in so far as we know has little func- 
tional value. The allantois has but little respiratory and excretory sig- 
nificance. Its work is practically to bring the embryo in relation to its 
food supply. It is the chorion which becomes the principal organ by 
which nutritive material and excreted substances between maternal and 
embryonic circulations take place. 

It is this connection between mother and embryo which brings about 
the formation of what is called the placenta. All mammals which 
develop a placenta — that is, all mammals except those which lay eggs — 
are known as placentals. 


The detailed development of mammals must be left to much larger 
volumes than this one, especially as so many variations occur even in 
quite similar groups of animals. 

Fig. 359. 

Diagrams illustrating the development of the blastocyst and formation of the 
placenta in^ Mammalia. A, a blastocyst at the end of segmentation; B, an older 
blastocyst, in which a cavity has appeared to one side of the inner mass of cells; 
C, a later stage, showing the formation of the trager and growth of the yolk 
epithelium around the yolk cavity; D, formation of lacunae in the trager and 
commencement of the embryo; E, further development of the trager, the mesoblast 
has split and the amnion and extra-embryonic ccelom are formed; F, longitudinal 
section of uterus, showing the position of the embryo in a pit in the uterine wall. 
G, longitudinal section of a later stage, showing the obliteration of the _ old 
lumen and formation of a new lumen in the uterus, all, allantois; am, amniotic 
cavity; cce, embryonic coelom; ec, epiblast; eec, extra embrj'onic coelom; ek, 
embryonic knob; em, embryo; gl, uterine glands; hy, hypoblast; im, inner mass of 
cells; lac, lacunae in Trager; lu}, original lumen of uterus; lu^, secondary lumen 
of uterus; nch, notochord; ng, neural groove; sbm, thickened sub-mucous layer of 
uterus; tr, trager; tro, trophoblast; yk, yolk-sac; yk.e, yolk epithelium. In all the 
figures the trophoblast is shaded with dots, and the embryonic mesoblast is represented 
in black. (After Bourne.) 

Mammalian Embryology 203 

But as the student must know the placental animals in order to 
make the most of his study in Comparative Anatomy, it is essential that 
he at least obtain a clear and accurate understanding of the two principal 
types of placental formation. 

In the first place, the placenta may be defined as consisting of all 
structures affecting nutritive, respiratory, and excretory interchanges 
between the embryo and its mother in viviparous animals. It is evident, 
then, that the placenta must form in the region where the trophoblast 
comes in contact with the uterine mucosa, and that the trophoderm 
itself, plus the vascularization in the yolk-sac, allantois, or chorion, will 
be the elements from which the placenta is developed. (Figs. 358, 359.) 

At this stage the student must review the chapter on the develop- 
ment of the extra-embryonic membranes in the chick. 

The rabbit is often used as an example of a form of mammalian 
embryology which can be contrasted with the embryological develop- 
ment of the chick. In the rabbit the extra-embryonic membranes develop 
quite like those in the chick, except that the point of fusion of these 
membranes consists of only a small knot, whereas in the chick this 
fusion takes the form of an elongated seam. In both rabbit and chick 
the tail-fold grows more rapidly than the head-fold. 

In man, where entypy takes place (that is, where the trophoblast 
remains continuous about the entire blastoderm), the extra-embryonic 
membranes do not grow as in the rabbit and chick, but by a splitting of 
the ectoderm to form the beginning of the amniotic cavity. 

The forming of the extra-embryonic membranes in man quite nat- 
urally causes the embryo to remain connected with the blastodermic 
wall by a body-stalk (Fig. 358, B). The separating of the ectoderm 
immediately above the embryo to form the amniotic cavity causes the 
embryo to form the floor of this cavity, while the trophoblast forms the 
roof. The sides, or walls, of the cavity meet the embryo at the edges of 
the embryonic shield. 

But, whether the amniotic cavity is formed as in the rabbit or as in 
man, the walls of the cavity extend ventrally until they surround the 

The yolk-sac and the yolk-stalk, as well as the allantois, although 
quite small in man, are pushed into this body-stalk or umbilical-stalk. 
The amniotic cavity grows large in man and contains from one-half to 
one liter of liquor amnii. 


The open space on the interior of the mammalian blastodermic vesi- 
cle is supposed to represent the yolk-sac (Fig. 359, G) of such animals 
as the chick and the frog; and, as this open space is relatively very large, 
the yolk-sac occupies the main portion of the early mammalian blasto- 
dermic vesicle. The cavity of the vesicle opens into the mid-gut region 

204 Mammalian Embryology 

by the broad yolk-stalk just as with the chick. Its wall is separated from 
the chorion by the extra-embryonic coelom — also called the exocoelom. 
(Fig. 359, E.) 

The amnion and chorion are formed from somatopleure, while the 
yolk-sac is formed from splanchnopleure. 

The blood vessels and the sinus terminalis arise in the yolk-sac of 
the rabbit just as they did in the chick. 

In the higher primates, including man, the yolk-sac never fills the 
entire blastodermic vesicle and is very slow to grow. In fact, during 
the first month it has a diameter about the length of the embryo, and 
after increasing this diameter to a little over a centimeter, it decreases in 
size. The yolk-stalk is formed, however, and elongates considerably to 
enter the proximal end of the umbilical cord. 

The amniotic membrane now expands and pushes against the exo- 
coelom until that is eliminated and the yolk-sac disappears in the pla- 
cental region. The yolk-stalk itself becomes a solid cord during the 
second month. However, the proximal end sometimes remains open. 
In such a case it appears as a diverticulum from the intestine and is 
called Meckel's diverticulum. 


This structure also varies in size to a considerable extent, from 
filling the entire exocoelom as in the lower primates such as the Lemurs, 
to occupying but a small portion of the umbilical cord as in man and the 
higher primates (Fig. 359, G). 

The early development of the allantois in the mammals is quite 
similar to that in the chick, but its later development is varied, the 
variation being ascribed to the changed conditions brought about by the 
formation of placental structures. 

The later history of the allantois is limited to the placental struc- 
tures only. In the rabbit the allantois extends into the exocoelom and 
comes in direct contact with the chorion in the region where chorion 
and uterus unite. It thus lies in the direct pathway of connection be- 
tween mother and offspring. Blood vessels now develop in the allantoic 
mesoderm to form the umbilical arteries and the umbilical veins, and it 
is through these allantoic blood vessels that the embryonic circulation 
is related to the placental circulation. 

In man the development is quite different ; for, here there is nothing 
which interrupts the connection of chorion with the maternal tissues. 
The way in which the body-stalk develops in man has been described 
already. This is often said to be equivalent to a modified allantoic stalk. 
There is, therefore, in man, no true allantois as a free vesicle. Only a 
small tubular outgrowth from the entodermal lining of the yolk-sac can 
be seen, and this outgrowth, in turn, is not distinguishable from the 
hind-gut. It extends into the body-stalk. As the embryo grows, and the 

Mammalian Embryology 205 

body-stalk extends, the allantoic stalk extends further along in the body- 
stalk as well, and so remains during foetal life (Fig. 360). 

As the ventral body-walls of the embryo are formed and approach 
each other, the proximal end of the allantoic stalk becomes the urinary 
bladder and the beginning of the urogenital sinus. From the bladder 
region to the body-wall it is reduced as a mere solid strand of connective 
tissue known as the urachus. 

Vascularization is quite alike in the various mammalian forms. 

The development of the placenta depends upon the manner and 
type of implantation, which in turn causes different relationships between 
the growing embryo and the maternal tissues. 


We have been describing the embryonic placenta. Now we shall 
describe the maternal placenta. There is a change which takes place in 
the lining of the uterine walls when the trophoderm unites with the 
uterus. The uterine lining which bulges out into the uterine cavity to 
cover the blastoderm, is called the decidua capsularis (formerly decidua 

reflexa), while the uterine lining at 
the point where blastoderm and 
uterus unite is called the decidua 
basalis or decidua serotina, the re- 
maining portion of the lining being 
known as the decidua vera (Fig. 

The chorion is at first composed 
of an inner mesodermal layer and 
an outer epithelial layer (this latter 
being called the trophectoderm). 
From the trophectoderm there de- 
velops an outer syncytial layer 
AT ^- t .• ^'/- ^f^-u u ^ which is called the trophoderm. It 

Medial section of early human embryo, . . . ^ 

(After von Spee, Kollmann.) IS thlS trOphodcrm which lUVadcS 

the maternal tissues. Large lacunae of blood are formed in the maternal 
tissues by the syncytial tissue directly, or by the rupture of the blood 
vessels which are under great pressure in this region. 

The trophoderm then thickens at intervals and forms little villi or 
finger-like projections, and the chorionic mesoderm grows out into these 
villi so that there is a branching of the primary villi into secondary villi 
or true villi (Fig. 358). 

In the meantime the blood lacunae run together and surround and 
bathe the villi, while the trophoderm, which began as a spongy network, 
is now a continuous layer covering the entire outer surfaces of the villi 
and chorion. 

Branches of the umbilical vessels develop in the mesoderm of the 


Mammalian Embryology 

chorion and villi. This means that there are noAv two layers of epi- 
thelium covering the mesodermal core of all the villi, and that it is in 
these villi that the chorionic circulation of the embryo is established. 

The blood vessels of the uterus open into the little blood-lacunae, 
which is another way of saying that the syncytial trophoderm, which 
covers the villi, is bathed in maternal blood. This is where the nourish- 
ment of the embryo takes place. The maternal blood itself does not pass 
into the developing embryo. 


C«,/ Cor<J 

Fig. 36L 

Diagram to show relationship o£ mammalian embryo and maternal membranes. 

At first the villi cover the entire surface of the chorion, but in man, 
after a few weeks, the villi located away from the point of attachment 
begin to degenerate and finally leave that portion smooth. This smooth 
region is called the chorion laeve, while the attached portion, which 
retains the villi, is known as the chorion frondosum (Fig. 362). It is 
the chorion frondosum, together with the decidua basalis, which con- 
stitutes the placenta. And it is the chorion frondosum to which the 
embryo is attached by the body-stalk which later comes to be called the 
umbilical-stalk or umbilical cord. 

Mammalian Embryology 


The decidua basalis forms what is called the maternal placenta, and 
the chorion frondosum the foetal placenta. 

The decidual membranes and 
their attachments form the after- 
birth. This afterbirth consists of 
amnion, chorion, decidua vera, 
placenta, and a part of the decidua 


As the body-stalk becomes 
longer and longer, finally reaching a 
length of some fifty centimeters, 
there must be some circulatory con- 
nection between the embryo and the 
chorion frondosum. This connec- 
tion is brought about by the development of four blood vessels, two 
veins and two arteries, known as the umbilical vessels or allantoic vessels 
(Fig. 363). The two veins push their way into the embryo to open into 
the heart. The arteries likewise grow in the same direction as do the 
veins, but connect with the dorsal aorta. Their distal ends extend 

Fig. 362. 

Human Embryo. Age seven weeks. (From 
Kollmann.) cf, chorion frondosum. cl, chorion 

Fig. 363. 

1 to 6, Diagrams representing six stages in the development of the foetal 
membranes in a mammal. 

The ectoderm is indicated by solid black lines; the entoderm by broken lines; 
the mesoderm by dotted lines and areas. (After Kolliker.) 

7, Diagram of nurture of young through embryonic membranes, g, gill circula- 
tion of embryo; h, heart; i, dorsal aorta; ;, postcava; k, allantoic artery; 
/, allantoic vein; m, indicating the course of the blood of the mother, parallel 
to n; n, that of the embryo; n, umbilical cords; w, wall of uterus. (After Needham.) 


Mammalian Embryology 

through the body-stalk into the villi to connect with the vascularization 
there established. 

The two veins later fuse, so that a cross section of a mature umbilical 
cord (Fig. 364), shows two arteries and a single large vein. 

Aa. umbilicalGs 

V. umbilicalis 

V. umbilicalis 

A. umbilicalis 


Ductus omphaloGntencus 

Ductus omphalo- 
eniericus (vitellinus) 



Fig. 364. 

I, Umbilical cord of human embryo at three months. 

II, Same at birth. (After Corning.) 

In addition to the umbilical vessels just mentioned, the yolk-stalk 
(in the early stages only), and the allantoic stalk can be seen in cross 
sections of the cord, while the cord itself is filled with a mesenchymal, 
mucous-like substance, called Wharton's jelly. 

The cord is twisted and is attached to the umbilicus or navel of the 
foetus and to the placenta. The outer covering of the umbilical cord is 
a layer of ectoderm which is continuous with that of the amnion of the 

The following table shows the relative increase in size and weight 
of the human embryo and foetus throughout the period of gestation : 


Ovum (estimated) 0.000004 grm. 

23 days 0.04 

56 days 3.0 

84 days 36.0 

112 days 120.0 

140 days 330.0 

168 days 600.0 

196 days 1000.0 

224 days 1500.0 

252 days 2200.0 

270 days 

280 days 3200.0 

C. H. 


C. R. 

2.5 mm. 

2.5 mm 



















^C. H.=Length as measured in a straight line from the crown of the head to the heel. 
C. R.=Length from crown to rump or sacral flexure. 

Mammalian Embryology 


If the student has thoroughly mastered the subject-matter of this 
i^ semester's work in embryology he will not only 

be able to understand how a normal embryo de- 
velops, but he will also know how and why many 
and varying types of abnormal development 
occur by either mechanical or chemical injury of 
some kind, which injury may cause any portion 
of the embryo to stop growing, while other parts 
continue in the usual manner. Monstrosities of 
many kinds may thus be formed, and even in 
apparently normal individuals it is by no means 
rare for the surgeon to find individual internal 
organs underdeveloped or overdeveloped. All 
such deviations from the normal are of the 
utmost importance to the medical man, and it is 
only through a study of embryology that they are 
made understandable. 

Figure to illustrate the 
"vertex-breech" method of 
measuring human embryos. 
a-b, vertex-breech length of 
the embryo. 

Comparative Anatomy 



IN the study of Comparative Anatomy a method somewhat different 
from our study up to this moment must be brought into play. 
In the forepart of this book, the frog- was studied as a type-form 
of vertebrates, and the earth-worm as a type-form of annelids as well as 
of coelomates, and then, after each such type-form had been studied, it 
was compared with other forms likewise studied in the laboratory. 

Now we are to take an entire system in each of the leading groups 
of vertebrates and compare system by system, always reviewing the 
development of the particular system studied, and comparing such 
development with the development of the respective systems in both 
frog and chick', as shown in Part One (embryology) of this book. 

Three distinct points of view must be kept in mind in Comparative 
Anatomy, namely, those of: 

Structure (both gross and microscopic). 

Development (embryology). 

Comparison of organ systems. 

Just as in any account of man's history we attempt to study those 
races which we now consider as living under primitive conditions, 
believing that they will throw some light upon the problems that our 
ancestors had to overcome in order to bring about our present state of 
civilization, so, in Comparative Anatomy we attempt to study the 
so-called lower-animal life in order that this may throw light upon the 
development of our own bodies. This may be brought home the better 
by remembering that all higher forms of life practically possess all 
organs and system of organs which the lower forms possess plus an 
additional something. This does not prove by any means that any of 
the higher systems must have necessarily come from the lower. All it 
does mean is that all forms of animals, which walk on the earth, must 
have much that is similar. For instance, legs used for the same purpose 
in all animals must have muscles that will function alike ; because, 
regardless of what position we systematically assign these animals, 
they, by virtue of the fact that they walk, must necessarily have leg 
muscles, and having these, there must be a supporting structure for 
such muscles, so that the skeletal systems of walking animals will be 
closely akin. 

Comparative Anatomy proper, then, will consist of a comparison of 
the organ systems of four great groups of vertebrates. The classic 
examples used for such comparison are : 

The dogfish, as an example of a group of living organisms whose 
skeletal tissues are largely cartilaginous. 

214 Comparative Anatomy 

The turtle, as an example of the reptilia. 

The cat, as an example of the mammalia. 

The frog, as the classic example of the amphibian. This animal has 
already been studied in the early part of the course, but fnust be kept 
in mind so as to be compared with the above three types. 

It is usual to exclude the aves, because reptile and bird have so many 
structural similarities that the study of one suffices for that of the other. 
In fact the single v^ord Sauropsida has come into common biological 
usage as meaning both reptiles and birds. 

It is necessary, first, for us to have some conception of v^hat is 
meant by the phylum Chordata and to appreciate that there are inter- 
mediate types betv^een invertebrates and vertebrates. Such intermedi- 
ate types are known as pro-chordata. The pro-chordata and the verte- 
brata together form what zoologists call the phylum Chordata. 

The vertebrates possess a spinal, or vertebral, column which con- 
sists of a great number of similar portions, called vertebrae, arranged in 
a longitudinal series. In the early embryo of all vertebrata there appears 
a rod-like notochord. This probably serves as a sort of stiffening to the 
animal, and in this respect only is it similar to the spinal column proper. 
It is neither cartilage nor bone, and probably develops from the entoderm 
or mesoderm. As the spinal cord develops from the ectoderm, and the 
bones of the spinal column from the mesoderm, it will be seen that 
neither of these three just-mentioned portions are alike in either origin, 
function, or position. 

In all vertebrates the main nerve cord lies on the dorsal side, while 
in invertebrates it lies on the ventral side. 

There are certain groups of animals which possess no spinal column, 
yet, during the embryonic period have a notochord, a dorsal nerve 
cord, and a gill-slit apparatus (Figs. 313, 314, 315, 316). The classic 
examples of these forms are Amphioxus, Balanoglossus, and the tunicate 
or sea squirt, all of which are comparatively small in size and live in 
the sea. These forms are grouped together under the name of pro- 
chordata. Professor Patton of Dartmouth College has described a 
scorpion in which he is sure he has found a notochord. If he is correct, 
it will be seen that no classification of this kind is absolute, in that 
invertebrates of very early geologic times may have possessed such an 
embryological structure. 





NLESS this chapter is mastered, there can be no understanding 
of the textual matter which follows, as the scientific terms there 
used are all based on what this chapter contains. 


The Chordata possess a notochord at some time during their life's 
history (the notochord lying between the nervous system and the 
alimentary tract), a hollow central nervous system lying entirely on one 
side of the digestive canal, and pharyngeal slits extending from the 
pharynx to the exterior. 

The Chordata are divided into four sub-phyla, all of which develop 
a notochord during their embryonic period, though all do not later 
develop a bony vertebral column. 

The subdivisions of the Chordata (Figs. 313, 314, 315, 316) are as 
follows : 
Sub-Phylum I. Cephalochordata (Adelochorda, Fig. 312). 

The notochord runs only up to the head proper in most chordates, 
but in the Cephalochordata, of which Amphioxus is the classic example, 
the notochord extends to the very anterior end of the body. Amphioxus 
is fish-like in form and is used as an example of the most primitive form 
of the chordates. It will be remembered that there was reference made 
to the simplicity of the embryology of Amphioxus in the early part of 
this book. 

Amphioxus has no skull or vertebral column. The pharyngeal slits 
are quite numerous. The true scientific name of Amphioxus is Branchi- 
ostoma. In popular language it is often called lancelet, on account of 
its sharp, lance-like appearance. 
Sub-Phylum II. Urochordata (Tunicates, Figs. 312, 313). 

This group possesses a notochord only in the caudal region. The 
young are tadpole-like, and there is a metamorphosis converting the 
tadpole into a sac-like structure. 

Order 1. Larvacea (Appendicularia), free-swimming forms with 
permanent tail. 

Order II. Ascidiacea (Tunicates or Sea-Squirts), fixed forms with- 
out tail in the adult. 

Order III. Thaliacea (Salpians), free-swimming forms without tail 
in the adult. 

The neurenteric canal is permanent. 

216 Comparative Anatomy 

Sub-Phylum III. Hemichordata (Fig. 314). 

A rather doubtful form. There is a projection from the mid-dorsal 
region of the digestive canal which looks somewhat similar to a noto- 
chord. These animals have a collar and a proboscis. 

Order I. Enteropneusta, which include worm-like forms such as 

Order II. Pterobranchiata, sessile, tube-dwelling forms such as 

Cephalodiscus, and Rhabdopleura. 
Order III. Phoronida, tubed forms such as Phoronis (Fig. 199). 

Sub-Phylum IV. Vertebrata (Craniata). 

1. The vertebrates show their segmentation in the adult form only 
on the interior of the body, as for example, the metameric arrangement 
of myotomes, sclerotomes, etc. 

2. A cuticular skeleton is absent, but there may be cornifications 
of the epithelium, or ossifications in the dermal regions, such as the 
scales of fishes, etc. 

3. An axial skeleton is present, consisting of skull and vertebral 

4. There are two kinds of appendages supported by the axial 
skeleton, namely, the unpaired fins (which occur only in fishes and 
Amphibia), and the paired appendages (anterior and posterior), which 
are usually present. 

5. The central nervous system is dorsal in position. The brain 
itself consists of five parts : the cerebrum, "twixt-brain," mid-brain, 
cerebellum, and medulla oblongata. 

6. Of the sensory organs, the eyes and ears are the most highly 

7. The respiratory organs arise from the entoderm of the pharynx. 
Pharyngeal slits are present in the embryo. In terrestrial animals these 
pharyngeal slits are later functionally replaced by lungs which develop 
from the hinder portion of the pharynx. 

8. The heart lies ventrally in the pericardium. In gill-breathing 
species it contains only venous blood, but in lung-breathing animals it 
is divided into venous and arterial halves. The circulation is closed. 

9. The sexes are usually separate, while in most species the excre- 
tory (nephridial) system forms the ducts for the reproductive (genital) 

10. Reproduction is strictly sexual. 
The classes of Vertebrata are as follows : 

Class I. Cyclostomata (Fig. 366). 

These are the round-mouthed eels without a lower jaw. Examples 
are the lampreys and hagfishes. It is in this group that we find the only 
vertebrate parasites, 



There is a primitive skull, but no true vertebrae (only bony arches). 
Paired fins, true scales, and teeth are lacking. The gill-pouches are 
saccular and the nose is unpaired. 

Sub-Class I. Myxinoidea (Fig. 366). 

These are the "hag-fishes" or ''borers" which give off a slimy. 

Bdellosfoma domheyi (Pacific ha^fish) 

Petromyzon marinus (sea lamprey) 

Fig. Z66. 

Cyclostomes. The light openings along the sides are mucous canals, the dark 
ones are branchial openings. 

mucous jelly when captured, 
name of Myxinoidea. 

It is from this fact that they receive their 

Sub-Class II. Petromyzontia (Fig. 366). 

These are the lampreys. They live in both salt and fresh water. 
The myxinoids attack principally dead and disabled fishes, but the 
petromyzons attack decidedly active fish much larger than themselves, 
attaching themselves to their host and making great inroads with their 
rasping tongues. 

Class 2. Pisces (Gnathostomata). All fish having true lower jaws. 

Fishes are distinguished from the Cyclostomes not only by having 
true lower jaws but also by having a vertebral column (amphicoele 
vertebrae, Fig. 404), by having scales, paired pectoral and pelvic fins, 
and paired nostrils. They breathe by gills and have a heart with venous 
blood therein only, although the heart has auricle, ventricle, sinus 
venosus, and some have a conus arteriosus. 

Sub-Class I. Elasmobranchii. 

These are the sharks and their near relatives. They have a car- 
tilaginous skeleton, usually a heterocercal tail, placoid scales (thornlike), 
but in Mustelus (the dog-shark, which is used in the laboratory), pointed, 
overlapping scales. There are five to seven slit-like gill-openings on 
each side. The eggs are few and hatched within a sac inside the body. 
The skates belong to this group as they are merely flattened out sharks. 


Comparative Anatomy 

There are various extinct orders and sub-orders of elasmobranchs, 
but v^e shall deal only with two orders and two sub-orders. 

Order Plagiostomi. 

Sub-Order I. Selachii (twelve living and three extinct families of 

sharks and dog-fishes, Fig. 367). 
Sub-Order II. Batoidei (Saw-fishes, skates, rays and torpedoes, 

seven families. Fig. 367). 

Chiniaera monslrosa . . , , . > 

Eaia erinacea (common skate) 

Fig. 367. Elasmobranchii. (A, after Goode; C, after Glaus.) 

Order Holocephali (Chimaera, Fig. 367, one living and three extinct 


The Holocephali are very grotesque looking animals and are of 
great antiquity. There are peculiar grinding plates in the mouth instead 
of teeth. 

Sub-Class II. Teleostomi. (The tru^ bony fishes.) 
Skeleton partly or entirely bony, a single gill-opening on each side 
leading to gill-arches on which there are gill filaments. There is also 
a swim-bladder which may disappear with age. 

In the higher forms where the skeleton is entirely ossified, the pelvic 
girdle approaches the pectoral one, so that the pelvic fins may be directly 
beneath the pectoral fins. It is the approach of the girdles and fins 
which is used in classifying fish, because this is supposed to show dif- 
ferent degrees of specialization. 

The position of the fins in the higher fishes is supposed to furnish 
evidence to show that amphibians and higher fishes are not closely 
Order I. Crossopterygii. 

Sub-Order I. Osteolepida. (Four extinct families.) 
Sub-Order II. Cladista. Polypterus and Calamichthys are the 
usual examples. (Fig. 368.) 

Order II. Chondrostei. (Five extinct and two living families.) 
These include the paddle-fishes and sturgeons (Fig. 368). 



Acipenser sturio (sturgeon) 

Anna calva (bow fin J 

Fig. 368. Ganoids. 

In B, e.g., Large external gill of the hyoid arch; Pc, pectoral fins; Pv., pelvic 
fins. The larva is drawn in a very characteristic attitude. 

In C note the elongated snout, the barbules bounding the ventral mouth, the 
operculum covering the gills, the rows of bony scutes, the markedly heterocercal tail. 

D, Ventral and side view. 

F, Amia calva (Bow fin), c.f., caudal fin; d.f., dorsal fin; pct.f., pectoral fin; 
pv.f; pelvic fin; v.f., ventral fin. (B, after Budgett; D, after Goode; E, after 
Tenney; F, after Giinther.) 

Order III/ Holostei. (Six extinct and two living- families.) 
These include the bow-fins and gar-pikes. (Fig*. 368.) 

Order IV. Teleostei. 

Sub-Order I. Malacopterygii (21 families). 

These include tarpons, herring, salmon, etc. (Fig. 369.) 

Sub-Order II. Ostariophysi (six families). 

These include carp, tench, cat-fishes, etc. (Fig. 369.) 

Sub-Order III. Symbranchii (two families). 

A small group of eel-like fishes having characteristics of both 

Ostariophysi and Apodes. 
Sub-Order IV. Apodes (five families). 
These are the eels. (Fig. 369.) 
Sub-Order V. Haplomi (fourteen families). 
These are the pickerel, killifishes (mud-minnows, etc.). 
Sub-Order VI. Heteromi (five families). 
These are the Fierasfer, etc. (Fig. 370.) 
Sub-Order VII. Catosteomi (eleven families). 

These are the stickle-backs, pipe-fishes, sea-horses, etc. (Fig. 370.) 
Sub-Order VIII. Percesoces (flying fishes), (twelve families). 
These include the Belone, sand-eels, rag-fishes, etc. (Fig. 370.) 

*The student will meet with the term "Ganoid" in his reading. This merely refers to a shiny 
scale. In the United States the gar-pike (^Lepidosteus) found in the Mississippi Valley, is commonly 
mentioned, although older writers made a distinct grouping of Ganoids, consisting of Orders I, II 
and III, using the African Polypterus as the classic example. In Lepidosteus, Ganoid scales have 
a sort of peg and socket arrangement. 


Comparative Anatomy 

Fig. 369. Teleostei. 

A, Brook Trout, a sub-genus of the Salmon family, a.L, adipose lobe of pelvic 
fin; an., anus c.f., caudal fin; d. f. 1, first dorsal fin; d. f. 2, second dorsal or 
adipose fin; /./., lateral line; op.,^ operculum; pct.f., pectoral fin; pv.f., pelvic fin; 
v.f., ventral fin. (A, after Vardine; B and C, after Goode; D, from Bull. U. S. 
F. C. 1895.) 

Sub-Order IX. Anacanthini (three families) 
These are the cod, etc. (Fig. 370.) 

Hippocaiiiiius barboun (sea horse) 

^ ^ 






















Uadus inonhiia (cod) 

Exoiiaules (jUbeili (flymg fish) 

Fig. 370. Teleostei. 

A, Fierasfer acus penetrating anal openings of holothurians. D, an, anus; c.f., 
caudal fin; d.f.l — 3, dorsal fins; m.x., maxilla; pct.f., pectoral fin; pmx., pre- 
maxilla; pv.f., pelvic fin; v.f. 1 and 2, ventral fins. (A, after Emery; B, after 
Bull. U. S. F. C. 1907; C, after Jordan and Evermann; D, after Cuvier.) 



Sub-Order X. Acanthopterygii (78 families). 

These include a great majority of our more common fishes, such as 

perch, bass, mackerel, flounders, gobies, shark-suckers, climbing 

perch, etc. (Fig. 371.) 
Sub-Order XI. Opisthomi (one family). 
These are the eel-like fishes. 

•/v' ? 

i -/ /. 



Fh,l ,i,!,nf< lini'iiiix (,,u{<->'ii.d fiutindu 

pioihii itHH-u''ifii<i (ijorcupine fislt) 

LupliiitH i/isi'ufur 

■fish itfj-frog or amjler) 

Fig. 371. TeleosteL 

B, Dissection of head of Climbing Perch to show accessory respiratory organ; 
F, normal and G, inflated porcupine fish. (A and E, after Cuvier; B. F and G, 
after Giinther; C, D, after Baskett.) 

Sub-Order XII. Pediculati (five families). 

These are the Anglers, Bathymal Sea-Devils, etc. (Fig. 371.) 

Sub-Order XIII. Plectognathi (seven families). 
These include the file-fishes, trunk-fishes, pufifers, porcupine fishes 
and sun-fishes. (Fig. 371.) 

Sub-Class III. Dipneusti (Dipnoi). The Lung-Fishes. (Fig. 372.) 
(Two extinct and two living families.) 


Comparative Ana'jomy 

These include the Neoceratodus, Protopterus and Lepidosiren. 
The skeleton of lung-fishes is largel}^ cartilaginous, but there is a 
tendency toward ossification. The swim-bladder serves as lungs. The 
very young individuals have long feather-like external gills. 
Appendix to the True Fishes. 

I. Palaeospondylidae (one family between cyclostomes and fishes). 
II. Ostracodermi (three orders of eight families, mostly armored 

III. Antiarchi (one family of mailed fishes). 

IV. Arthrodira (one family of mailed fishes). 

Neoceratodus fosien. (Australian lung-fish) 

Protopterus annectens (African lung-fish) 

Fig. 372. Dipneusti. 

Lepidosrren lart'f, 

In C, snt., sensory tubes; /./., lateral line;, external gills; pel., pectoral 
fin; op, operculum. In D and E, eg., external gills; Pc, pectoral fin; Pv., pelvic 
fin. (A, after Gunther; B, after Claus; C, after W. N. Parker; D, after Budgett; 
E and F, after Graham Kerr.) 

It is well to note that 172 families of the 226 families of true fishes 
are members of the order Teleostei. 

Of the Elasmobranchii there are 23 families now in existence and 
nine extinct. 

The ganoids and dipnoi number 22 families. 


Contrasted with fishes, the amphibia have pentadactyl appendages, 
while contrasted with reptiles, they possess double occipital condyles. 
There are external gills in the larvae, though these do not always per- 
sist. The adults breathe by lungs. The heart consists of two auricles, 
one ventricle, a conus arteriosus, and a sinus venosus. 

Sub-Class I. Stegocephali. 

These are the extinct amphibia, many of which attained consid- 
erable size. 



Sub-Class II. Lissamphibia. (About 1,000 species, nearly 900 of which 

are frogs and toads. Figs. 315, v376.) 

Order I. Apoda (Gymnophiona) Limbless Amphibia. (Fig. 373.) 

These are also called caecilians and sometimes "blind-worms." 

They are without limbs or limb-girdles. They burrow in the earth and 

are found in warm climes. The cranium is like that of the reptile in 

outward appearance, but the bones which constitute it are the same as 

those which go to form any amphibian cranium. The skin is smooth 

and slimy with many ring-like folds. There are as many as 200 to 300 

vertebrae in some species. The eyes are rudimentary and probably 

functionless. Between eye and nose, a feeling organ protrudes which 

serves to guide the animal. Some are oviparous, while others are 


Fig. 373. Apoda. 

Ichthyophis glutinosa. 1, nearly ripe embryo, with gills tail-fin, and with 
considerable amount of yolk; 2, a female guarding her eggs, coiled up in an under- 
ground hole; 3, a group of newly laid eggs; 4, a single egg, enlarged and schematised 
to show the twisted albuminous strings or chalazae inside the outer membrane, 
which surrounds the white of the tgg. 5. Caecilia, emerging from burrow. (After 
P. and F. Sarasin.) 

Order II. Urodela (Tailed Amphibia). (Figs. 315, 374.) 
These are the mud-puppies (Necturus), salamanders, newts, and 
efts. Many authors call all urodeles with adult external gills, Perenni- 
branchiata, though the following grouping is the more common : 

Family I. Amphiumidae (Fig. 374). 

This family is without external gills in the adult stage. There are 
only two genera, Cryptobranchus and Amphiuma (Fig. 374). 

Cryptobranchus allegheniensis (Fig. 374), is the well-known ''hell- 
bender" of the Eastern United States. 

Cryptobranchus japonicus is the giant salamander Of Japan. 


Comparative Anatomy 

Amphiuma (Fig. 374) has only one species which ranges from 
Carolina to Mississippi in our Southeastern States. This is known as 
Amphiuma means, and is eel-shaped with much reduced limbs and a 
small pair of inconspicuous gill-clefts guarded by skin-flaps. Some of 
these animals are three feet in length, living in swamps and muddy 
water. The female protects the eggs by coiling about them. 
Family II. Salamandridae. (Salamander and Newts.) (Fig. 374.) 

These animals have no gills in the adult stage. Practically three- 
fourths of all tailed amphibia belong to this family. 

The most common type is the Desmognathus fuscus (Fig. 374). 
The female coils about the eggs when laid. The young, after hatching, 
look quite like adult forms. 

Amblystoma tigrinum or "tiger salamander" (Fig. 374) is very com- 
mon in North America. It has large yellow spots which may merge 
into broad stripes or bands. The ground color is black. It may b)e 

Fig. 374. Salamandridae. 
In C, 1, Female; 2, Male at the breeding season with well-developed frills. 
E, Desmognathus fuscus (American newt). Female with eggs in underground hole. 
(A, after Molder; B, from Cambridge Natural History; C, after Gadow, E, after 

found in damp places under stones and logs, or even in cellars of houses. 
For various reasons, it is one of the classic forms used in the laboratory. 
One laboratory value is that it is an animal which becomes sexually 
mature while still in the larval stage, a condition called paedogenesis 
or neoteny. 

Another very interesting fact is brought out in the life of the larval 
forms of Amblystoma. The larva itself is called Axolotl, and was for- 
merly considered to be a fully adult form. It is quite common near 



Mexico City. However, when some of the Axolotls were taken to 
Paris, and kept in aquaria, they metamorphosed into regular, full-fledged 
Amblystomas. Not only this, but some of them could be made to revert 
back to the larval Axolotl form. 

Salamander maculosa, commonly called the ''spotted" or "fire sala- 
mander" is the most common of the European salamanders. 

Salamandra atra is much darker than S. maculosa and is found in 
the Alps at altitudes from 2,000 to 9,000 feet. This animal is interesting 
in that it produces only two young at a time, which, while still in the 
uterus, feed upon the surrounding eggs and pass through their entire 
metamorphosis before being born. 

Kammerer claims that S. atra will change to S. maculosa if brought 
to lowland waters and then after being kept there for several genera- 
tions, and later returned to the higher altitudes, they will retain the 
breeding habits acquired as the lowland type. This fact has led some 
authors to insist that here is a case of acquired characteristics being 

Diemictylus viridescens is the "vermilion spotted eft" or newt. It 
takes several years to reach the adult form. For three years it lives in 
water and has external gills. During this time it is green in color. Upon 
leaving the water, it becomes yellow with vermilion spots, and at the 
breeding season returns to water and again becomes green. 

Triton cristatus (Fig. 374) is the "crested newt." The male has a 
decided crest during the breeding season. 
Family III. Proteidae. (The Mud-Puppies), Fig. 375.) 

These have three pairs of fringed external gills throughout life, and 
some authors call them perennibranchii. 

There are only three genera, with a single species each. Two of 
these genera occur in America and one in Europe. 

(mud- puppy J 

Sin>n JacerluKi (mud-eel) 

Fig. 375. Proteidae and Sirenidae. 
(After Chapin and Rettger.) 

Necturus maculatus is the common American "mud-puppy." It is 
assumed that this may be an animal which has remained in the larval 


Comparative Anatomy 

Proteus anguineus (the Germans call them *'olms") are blind cave 
mud-puppies nearly white in color. But, if brought into the light, they 
become at first grayish and then jet-black. 

Typhlomolge rathbuni is a form quite like Proteus, and is found 
in subterranean caves and sometimes is brought up from deep artesian 
wells. They are found in Texas. 
Family IV. Sirenidae. (The Sirens), (Fig. 375.) 

These have three pair of permanent fringed external gills, and the 
body is eel-like. There are no hind limbs. There are two genera, each 
with a single species. 

Siren lacertina, commonly called the "mud-eel." It may reach a 
length of thirty inches. It is black in color dorsally and lighter ven- 
trally. It is found in the southeastern part of the United States. 

Pseudobranchus striatus is much smaller than Siren, hardly ever 
reaching a length of more than seven inches. It has one pair of gill- 
clefts and only three fingers. There is a broad yellow band along each 
side. It is assigned the lowest place among the urodeles. 
Order III. Anura (Tailless Amphibia), (Figs. 315, 376). 

These are the frogs and toads. 

Sub-Order I. Aglossa. 

These animals have no tongue. 

This group is not yet commonly known as it occurs only in South 
America and in Africa. 

Pipa americana (also known as the "Surinam toad," Fig. 376), has 
rather remarkable methods of carrying its eggs after they have been 
laid. There are holes in the back of the female into which they are 

Xenopus and Hymenochirus are the African genera. 

(flying tree-load of Borneo) 

Ahjlcs ohsieincans 
(ohstelrical load) 

Pljia americana 

Fig. 376. Anura. 

A, Pipa americana with young in skin pockets of back. C, Male obstetrical 
toad with string of eggs: (A, after Ludwig; B, after Wallace; C, after Claus.) 

Classification 227 

Sub-Order II. Phaneroglossa. 

These are the frogs and toads with tongues. There are seven 

The best known of these families are the Buffonidae, which are the 
common toads, and the Ranidae, the "true frogs." 

There is a peculiar species of toads in France and Switzerland called 
Alytes obstetricans (Fig. 376), in which the male takes the eggs when 
laid and wraps them around his hind legs, after which he deposits them 
in a hole in the ground. These eggs are then moistened by him with 
dew and taken out occasionally in the water. When the eggs are ready 
for hatching, he takes them all to the water and remains with them 
until hatching is complete. 

Class Reptilia. 

There are four orders of living reptiles. These are cold-blooded 
vertebrates, breathing by means of lungs throughout their life cycle. 
Lizards, snakes, crocodilians, and turtles come under the heading of 

The fossil records of the past show that the four living orders are 
but a small portion of the variations within this class, which have con- 
tinued their existence. 

In the Mesozoic era (Fig. 245, Vol. I), commonly called the "age of 
Reptiles," there have been found many skeleton-remains of immensely 
large Hzard-like animals. In fact, the name given to the largest of these 
animals of the past is Dinosaur which means "terrible lizard." 

Sphenodon pundaium 
Fig. 377. Reptilia. 

(.Sphenodon is considered the most primitive type of living reptiles.) (After Gadow.) 

There were many flying reptiles at that time, while Plesiosaurs 
lived in the water and had long paddles for swimming instead of legs. 

Reptiles with wings are called pterosaurs. Some of their fossil 
remains show these animals to have been twenty feet from tip to tip of 
wings when spread. It is assumed that these animals so overspecialized 
various parts of their body that, when great climatic and earth-changes 
came about, they could not cope with the new conditions. It has also 
been suggested that the eggs of many of these great animals may have 
been used for food by very small mammals, which caused the largest of 
all beasts to die out entirely. 


Comparative Anatomy 

The reptile most closely resembling extinct forms is thought to be 
Sphenodon (Fig. 377). It is confined to a few small islands off the coast 
of New Zealand and is hunted and eaten by the Maoris. It is also 
called "tuatara" and lives in burrows. Externally it looks like a lizard, 
though skeleton and viscera are quite unlike those of other living 

Order Chelonia. (Turtles and Tortoises.) 

These have a bony covering and toothless jaws. The covering 
consists of a dorsal or upper portion called a carapace, and a ventral 
plastron. These plates are soft in very young animals. The surface is 
covered with horny shields which Gadow believes to be phylogenetically 
older than the underlying bony plates. These latter do not correspond 
with the former in either number or position. 

There are two sub-orders. 

Sub-Order I. Athecae. 

These are without a true carapace 
sentative of this type, 

There is only one living repre- 
namely the Leather-back Turtle, known as 
Dermochelys (sphargis) coriacea. (Fig. 378.) 
Instead of the regular carapace, there are five 
dorsal, five ventral, and two lateral dermal 
plates. The tail is rudimentary and the limbs 
are large flipper-like paddles. Only large and 
very small specimens have ever been found. 
It is not known where they live between these 

Sub-Order II. Thecophora. 
These are the true turtles (Fig. 379) which 
are divided into two groupings. The first 
group is known as Cryptodira to which most 
turtles of the northern hemisphere belong. 
The head is retractile, and the pelvis is not 
fused to the shell. In Division 11 are the 
Pleurodira, representing a large group of the 
southern hemisphere. These do not retract 

the head but bend it sideways under the shell. The pelvis is fused to the 


The more commonly known American turtles belong to Division 
Cryptodira. The snapping turtles are members of the family Chely- 
dridae. The skunk or musk-turtle is a member of the family Cinoster- 
nidae, and the common pond tortoises are members of the famil}^ 
Testudinidae. The "tortoise-shell" turtle belongs to those commonly 
called ''sea-turtles" and is a member of the family Chelonidae. 

Sphargis coriacea 
(leather-hack turtle) 

Fig. 378. 
The only chelonian without a 
true carapace. (From Gadow.) 



Order Crocodilia. 

Crocodilia are charac- 
terized by well-developed 
limbs, long tail, fixed quad- 
rate bone, and teeth fixed 
separately in alveoli. The 
various extinct forms, how- 
ever, do not have all these 

There are only two fam- 
ilies of Crocodilia (Fig. 380). 
Family I. Gavialidae. 

There is only one living 
species of this family. It is 
called Gavialis gangeticus. 
It is found in the River 
Ganges and other large 
rivers of India. 
Family II. Crocodilidae. 

This family includes 
both old and new world 
crocodiles and alligators. 
The latter animals do not 
grow so large as the croco- 
diles. The alligator is dis- 
tinguished from the croco- 
dile by having a broad, 
rounded snout. 
Order Sauria. (Squamata.) 

These are the lizards 
and snakes. Their main dif- 
ferentiating characteristics 
are a movable quadrate bone 
which permits of a wide 
mouth-opening, a transverse 
cloacal aperture, and double 
copulatory organs. 
Division I. Lacertilia. (Lizards.) 

While normally the ordinary forms of lizards are scaly and have 
four well-developed legs, there are many species which do not have 
these characteristics. This latter type appear quite like snakes, but the 
bones of the skull always serve to distinguish them. Then, too, the 
lizards have no elastic ligament between the two halves of the lower 
jaw as snakes have. 

Fig. Z19. Chelonia. 

A, grows to weigh 800 pounds; B,_40 pounds C, 
smallest of marine turtles. This latter is the tortoise- 
shell turtle. (After Ditmars.) 


Comparative Anatomy 

There are three sub- 
orders (Fig. 381). 

Sub-Order I. Geckones. 
These are the primitive 
types having amphicoelous 
vertebrae (Fig. 404), and no 
bony temporal arches. They 
have dilated clavicles, sepa- 
rate parietals, eyes w^ith 
movable lids, and a broad, 
fleshy, protrusible tongue, 
w^hich is nicked at the end. 
The animals are usually 
harmless. The tail is loosely 
articulated and comes off 
when seized, although a nev^ 
one groves quite readily. 

Sub-Order II. Lacertae. 
Most modern lizards be- 
long to this group. Their 
vertebrae are procoelous and 
solid. The ventral portions 
of the clavicle are not 
There are cursorial types, arboreal types, volant types, an aquatic 
type, a fossorial type, and an ant-eating type, so-called from their vary- 
ing modes of life. 

The Gila Monster (Heloderma horridum) of our southwestern 
states is the only poisonous lizard known, while the monitor (Varanus 
salvator) grows to the greatest length, something like seven feet or 

Sub-Order III. Chamaeleontes (Chameleons). (Fig. 382.) 

These animals are the ones so well known on account of their 
ability to change color and the enormously long tongue by which they 
readily catch insects at a distance of some seven inches. 

Chameleons are highly specialized. The body is laterally com- 
pressed, the tail is prehensile, and the toes are parted in the middle so as 
to be used for grasping. They are found mostly in Madagascar. One 
species is found in southern Europe. 

Division II. Ophidia (Snakes). 

Snakes are really Sauria, or Squamata, in which the right and left 
halves of the lower jaw are connected with an elastic ligament which 
permits the mouth to stretch greatly. They are usually limbless or have 


Fig. 380, 
(After Baskett and Ditmars.) 



Anguis Imgilis (limbless lizard) 

Iguana tuberculaia (common iguana) 
Fig. 381. Lacertilia. 
(A and D, after Gadow; B, after Ditmars; C, after Shipley and MacBride.) 

Anolis principalus (American chamaeleon) 

Fig. 382. Chamaeleontes. 
(A, after Gadow; B, Ditmars.) 


Comparative Anatomy 

rudimentary limbs under the skin as has the python, 
out eyelids. 

The eyes are with- 

Class V. Aves. (Birds.) 

These are closely related to the reptiles. In fact, reptiles and birds 
are often grouped together as Sauropsida. They have a single occipital 
condyle as do the reptiles. The heart of birds is, however, divided into 
right and left halves. Birds are warm-blooded. There is a fusion of 
the bones of the manus and there is the formation of a tibio-tarsus and 
tarso-metatarsus (intratarsal joint). Feathers cover the body. 

Birds are commonly divided into : Ratitae, or "running birds," such 
as the ostrich, rheas, cassowaries, etc., whose sternum lacks a furcula 
(wish-bone) and a keeil (Fig. 418) ; and the Carinatae, or the "flying 
birds." These latter have the sternum keeled and the clavicles are 
united to form the furcula. 

There are two extinct groups which had teeth. 

Class VI. Mammalia. 

These are warm-blooded animals, having a covering of hair, two 
occipital condyles, and milk-glands in the female. 
Mammals are divided into two sub-classes. 
Sub-Class I. Prototheria, or egg-laying mammals. 

Order I. Monotremata, which consists of two families (Fig. 383). 

Echidna aculrnlu (sinny nnl.-ealer) 

Fig. 383. Monotremata. 

C. Echidna hystrix. I, lower surface of brooding female; //, dissection showing 
a dorsal view of the marsupium and mammary glands; t t, the two tufts of hair 
projecting from the mammary pouches from which the secretion flows; hm., 
brood-pouch or marsupium; cl, cloaca; g.m., groups of mammary glands. (A, after 
Shipley and MacBride; B, after Claus; C, after Haake.) 



Family I. Ornithorhynchidae. (The duck-bill of Australia.) 

There is no corpus callosum (Fig. 471), and the brain is the most 
primitive of all living mammals. 

The eggs, two or three in number, and covered with a hard shell, 
are reptilian in form and are laid in a nest of grasses. The heat of the 
mother's body hatches them. 

Family II. Echidnidae. 

These are the Australian Ant-Eaters. There is a temporary mar- 
supial pouch. Only one ^gg, about half an inch long, is laid at a time 
and placed in the marsupial pouch by the mouth of the mother. Here 
the young hatch in a very immature condition, the mother being obliged 
to remove the egg-shell after the young has come forth. The young 
Echidna obtains its food by licking the milk-like secretion exuding from 
the hairs in the pouch. 

Sub-Class II. Eutheria. 

These are the viviparous mammals which are divided into two 
divisions : 

Division I. Didelphia (Metatheria). (Fig. 384.) 

These are the marsupials. 

Order I. Marsupialia. Mammals having a pouch to carry their 
young which are born in a rather immature condition. There is usually 


/? ^^m 

^- "^^i^ 


Didelphys dorsigera (South American opossum) 

Petrogale xanthopus (rock wallaby 
with young in pouch) 

(A, after Vogt Specht; B, after Nicholson.) 

Fig. 384. Didelphia, 


Comparative Anatomy 

no placenta. Australia furnishes us with most Marsupialia, such as the 
kangaroo, wombat, phalangerer, pouched mole, and many other forms. 
The opossum is the only example in America. 

Division II. Monodelphia. (Placental Mammals.) 

The young are never carried in a pouch, but a true placenta nour- 
ishes the unborn foetus. 

Scalujj.'i ricqitndcus (coiitmon mole) 

Fig. 385. 
(After Coues.) 

Sorex vulgaris (common shrew) 

The placental animals are divided into the following sections: 
Unguiculates, Primates, Ungulates, and Cetacea. 

Section A, Unguiculates. (Clawed animals.) 

Order I. Insectivora, such as moles, shrews, and hedgehogs (Fig. 


Order II. Chiroptera, such as bats (Fig. 386). 

Order III. Carnivora, possess sharp teeth and claws. 

Under this heading come the 
cat (Felidae) and dog (Canidae) 

families, for example, and many 

Order IV. Rodentia are the 

gnawing animals. Rabbits, guinea 
pigs (Cavia), rats, mice, squirrels, 
etc., come under this heading. 

Order V. Edentata. This name 

means toothless, but the animals, 

with the exception of the ant-eaters, 

, belonging to this group do possess 

^ teeth. Different authors classify 

i the Edentata in various ways. The 

^ animals usually coming within this 

group are sloths, ant-eaters, and 

/ armadillos. (Fig. 387.) 

XnnUiurpiim collans (bal) \ <j / 

Fig. 386. Chiroptera. i Section B, Primatcs. (Mam- 

(After Sciater.) ..; ^als with uails.) 



Chotaciiii.'i duJatlijliis ftwo-tucd duik) 
TainaiiJmi Ichvdticlijhi [Tanaiiidaa anl-mier) 

Fig. 387. Edentata. 
(A and B, after Vogt and Specht; C, from Proc. Zool. Soc. 1871.) 

Order VI. Primates. Mostly tree-inhabiting animals, with nails 
on fingers and toes instead of claws or hoofs. The monkeys, which 
are to be included under this heading, are divided into Platyrrhine 
(broad-nostril) and Catarrhine (narrow-nostril) groups. The former 
are peculiar to the New World and the latter to the Old World. The 
higher apes belong to the Old World group. New World monkeys 
have a prehensile tail (Fig. 388) while no Old World monkeys possess 

Cebus li i/poh'.HCits 
( wJi iir-llM-oal.rd iifipajo u) 

Fig. 388. 
Note the prehensile tail so characteristic of New World Monkeys. 


Comparative Anatomy 

this. In the anthropoid, or manlike, apes (Simiidae), (Fig. 389), there is 
no tail at all. 

Section C, Ungulates. (Hoofed animals.) 

Order VII. Artiodactyla (even-toed ungulates), are pigs (Suidae), 
deer (Cervidae), giraffes (Giraffidae), cattle, sheep, goats (Bovidae). 

Artiodactyla are all terrestrial or mud-inhabiting animals, usually 
of large size, having hoofs on two or four toes. Their stomachs usually 
Iiave several chambers and are peculiarly adapted for an herbivorous 

Artiodactyla are often divided into two groups : 

liylobrites enielloxdes 
(dun-colored gibbon) 

Fan ( A n t h rope pit h ecus) 
troglodyfes (chimpanzpe) 

Simm satynis (orang-utan) 

Fig. 389. Simiidae. 

(A and B, after Flower and Lydekker; C, after Vogt and Specht; D. after 
Shipley and MacBride.) 




Bnluor^ ihi(ion<j n])i<jon(j) 

Martitfus ( untnaif'e or sm-coiv) 

Fig. 390. Sirenia. 
(A, from Brehm; B, from Ingersoll.) 

Group I. Suina. (Swine-like.) 

All the swine family come under this heading, including the hippo- 
potamus which is really an aquatic hog. 

Group II. Ruminantia. (Ruminants.) 

The animals belonging to this group swallow their food rapidly and 
later regurgitate it into the mouth for further chewing. Such animals 
are said to *'chew a cud." Camels, llamas, antelopes, cows, giraffes, 
goats and sheep belong here. 


Comparative Anatomy 

Order VIII. Perissodactyla. (Odd toed ungulates.) 

In this order, the animal walks on the middle digit' of fore and hind 
feet. The following three families make up the entire order: Equidae 
(horses, asses, and zebras) ; Tapiridae (tapirs) ; and Rhinocerotidae 

Order IX. Proboscidia. (Elephants.) 


Order X. Sirenia. (Sea-cows such as dugongs and manatees, Fig. 

Aquatic offshoots of ungulate stock. 

Hyrax abyssinicus (coneys or hyraces) 
Fig. 391. Hyracoidea. 

(From Lull after Brehm.) 

Order XI. Hyracoidea (Coneys, Fig. 391). 

Short-eared, rodent-like, primitive ungulates, usually living among 
rocks, although some are tree-inhabiting. 

Section D. Cetacea (Whales and dolphins, Fig. 392). 

Drl/jliinus delphis (dolphin) 

fiaheiin myslecetus (ivhahhone whale) 

Fig. 392. Cetacea. 

In D, a, upper arm; b, blow-hole; fa, forearm; h, hand;, small remains 
of pelvis, thigh, and leg; r, roof of palate; w.w., plates of whalebone; /, whalebone 
fringe. (A, after Flower and Lydekker; B, after Cuvier; C, after Sedgwick; D, 
after Holder.) 

Classification 239 

Order XII. Odontoceti (Toothed-whales). 

Examples of these are : sperm-whales, narwhals, beaked whales, 
porpoises, and dolphins. They have teeth but no whale-bone. They 
possess a single nostril or "blow-hole," and some of the ribs are two- 

Order XIII. Mystacoceti (Whale-bone whales). 

These are also called baleen whales. 

For convenience sake the following terms are often used: 


This is a name given to Cyclostomes, Gnathostomes, and Amphibia 
combined. The distinctive characteristics of the Ichthyopsida are that 
all animals belonging to this group breathe by means of gills at some 
period of their life's history. They are, therefore, aquatic vertebrates. 

They are sometimes called Anamniota or Anamnia, because they do 
not develop an amnion, and Anallantoida because they do not develop 
an allantois. 


This is the name given to birds and reptiles combined. The dis- 
tinctive characteristics of the Sauropsida are that all animals belonging 
to this group breathe with lungs and never develop functional gills. 
They are, therefore, terrestrial vertebrates. Sauropsida, together with 
the Mammalia, are called Amniota on account of their developing an 
amnion, and Allantoida on account of their developing an allantois. 


This is the collective name assigned to all four limbed animals, 
whether they are amphibians, reptiles, or mammals. 



IN examining an organism it is logical to first examine its external 
structure. It is thus the outer covering of the body which becomes 
our first object of study. In fishes we, therefore, study scales; in 
the frog and the human being, the skin; while on most mammals, fur; 
and on birds, feathers. Yet, whatever forms such external parts may 
assume, they are a covering of the body, and as such form what is called 
an integument ( ). This term includes the skin, 

or cutis, and all the structures derived from it. If an animal lives in 
water, the effect of water upon such covering must be considered; 
likewise, consideration must be given to whether an animal lives in a 
cold climate or in a warm, whether it lives in the air or burrows beneath 
the earth. All these things are bound to have modifying effects upon 
an animal's outer covering. 

Microscopically the integument of vertebrates consists of two 
layers: (Fig. 393), an outer, epidermis, which is the remainder of the 
ectoderm after the nervous system has been separated from it, and a 
deeper layer, the cerium, or derma, composed of mesenchyme which has 
been derived from the somatic portion of the somite. It is into this 
deeper structure that the nerves and blood vessels extend. 

Accessory organs are developed in both layers, but may begin 

growth in one and extend through the other. In all cases, however, 

each element of the accessory organs has a very definite place of origin. 

The integumental glands thus arise from the epidermis, though 

dipping down into the corium to receive a fibrous covering. 

Pigment usually develops in the corium and often then migrates 
to the epidermis, although it does sometimes develop in the latter layer 

Blood vessels (except in the mucous membrane of the pharynx of 
lungless salamanders) develop in the corium. 

Sensory nerve endings are quite freely distributed throughout the 
epidermis, but the more specialized forms remain in the corium, often 
pushed up into the epidermic zone in the form of papillae. The epider- 
mis is thus a bloodless, protective covering with but slight sensitiveness. 
All the more delicate structures are found in the lower layer or dermis. 
Both skin layers have the power to form hard parts, known as 

True bone, for example, develops from the corium, while horn and 
enamel originate in the epidermis. 

Horny structures, such as hairs or feathers (Fig. 394), are formed 

The Integument 


from the epidermis alone but dip down into the richly vascular corium 
to obtain nourishment. The dermal scutes of ganoids and the dermal 
bones of higher forms arise entirely within the corium. Teeth are com- 
posite structures composed of dentine, a hard sort of bone from the 
corium, overlaid with enamel from the epidermis. 

It is important that one does not confuse the term integument with 
mere portions of the integument ; for example, the epidermis is merely 
an outer histologic layer. The ectoderm is merely one of the germ 
layers from which both integument and the nervous system arise. The 
skin alone on such animals as have feathers, scales or fur, likewise would 
not be the integument, but both skin and its immediate outer covering 
would constitute such protective substance. The following schematic 
arrangement in man is that commonly used in medicine : 



1. Stratum corneum. 

2. Stratum lucidum. 

3. Stratum granulosum. 

4. Stratum mucosum. 

5. Stratum germinativum 

pighian layer). 


Secondary Epidermal Structures. 

" Hair. 
Nails, claws, hoofs, beaks. 
Exoskeleton. \ Feathers. 

Epidermal scales. 
Sensory nerve-endings. 

called Corium) 

1. Papillary layer, made up of 

dense connective tissue. 

2. Reticular layer, made up of 

looser connective tissue. 

Secondary Dermal Structures. 

Blood vessels. 
Lymph spaces. 
Dermal scales. 

Note: The exoskeleton of vertebrates consists of bone, horn, and 

Bone originates in the corium (mesodermal). 

Horn and Enamel originate in the epidermis (ectodermal). 


Comparative Anatomy 

In comparative anatomy, the epidermis in turn is divided into two 
layers, the lower one being known as the Malpighian layer or stratum 
germinativum (Fig. 393). Usually this layer rests on the corium and 
is nourished by the fluids from the corium. The cells, therefore, grow 
outward as they divide to form a second or outer layer, the stratum 
corneum. These outer cells come in contact with the surrounding 
media and are worn away almost as fast as new ones are added from 


Papilla dj- Oe*'^^^ 

T^e Ac c//q ^ A ayef^ 

C'*n sec /Vofj 

Fig. 393. 

Diagram of a section through the skin of a mammal to show various layers, 
hair, and sebaceous and sweat glands, 

below. If these outer cells come off in large sheets, we find such a con- 
dition as that of a snake shedding its skin. 

In land animals, the first layer of cells budding off from the Mal- 
pighian stratum seems to be a continuous sheet which is likely to be shed 
as a whole. This is called the periderm (Fig. 395). Older books call 
this the epitrichium, but as this word means ''above the hair" it is not 
accurate when it refers to reptiles and birds which have no hair. 

The Malpighian layer is that in and from which the glands of the 

The Integument 


skin are formed, while the corresponding part of the ectoderm con- 
tributes to such sensory structures as the nose and ear. 

The hair, nails, claws, feathers, and other outgrowths of the cutis 
come from the epidermis (Figs. 393, 394). Land animals usually have a 
thicker epidermis than those which live in water. The latter keep the 
outer portion of the body constantly moist and so show less of the hard- 
ened, or horny, consistency which is found in animals living in the air. 
The cerium lies directly beneath the epidermis and is connected by 
means of a loose layer of connective tissue with the deeper structures. 
The corium itself is a mass of fibrous connective tissue in which there 
is an intermingling of elastic tissue, blood vessels, nerves, smooth muscle 
fibers, etc. It is much thicker in mammals than in the lower vertebrates. 

It is the corium which is com- 
monly known as leather. In both 
epidermis and corium pigment 
cells may be found. These are 
mesenchyme cells loaded with 
pigment. They are frequently 
under the control of the nervous 
(sympathetic) system and can be 
altered in shape (chromato- 
phores), thus producing color 
changes, which as in the chamel- 
eons, may be very marked. 

If the epidermis becomes 
cornified, scales are produced. 
This takes place by certain cells 
in both corium and epidermis 
beginning to multiply in certain 
definite regions. These thicken- 
ings become future scales by the 
stratum corneum turning into a horny material. In snakes and 
lizards these scales, together with all of the stratum corneum (even the 
covering of the eye), are periodically moulted, the separation taking 
place at the surface of the stratum Malpighii. In turtles and alligators 
there is a gradual wearing away of the surface. 

Claws, hoofs, and nails are closely allied in their manner of growth 
to scales (Fig. 396). In fact, a claw is formed by two scales. The dorsal 
one is called the unguis, and the ventral the sub-unguis. The dorsal 
scale grows continually from a root and in mammals is forced over its 
bed. The unguis is curved both transversally and longitudinally, while 
the sub-unguis forms its lower surface. 

In the human nail, the unguis is nearly flat in both directions, and 
the sub-unguis is reduced to a narrow plate just beneath the tip of the 


Fig. 394. 

A diagram of a developing feather, highly 
magnified. der., Dermis; epi., epidermis; fol., 
follicle; fth., feather; Mp., Malpighian layer of 
epidermis; pap., papilla by the growth of whose 
epidermis the feather is formed. (From Shipley 
and MacBride.) 


Comparative Anatomy 

nail. In the hoof, the unguis is rolled around the tip of the toe, while the 
sub-unguis forms the "sole" inside it. The "frog" is the reduced ball of 
the toe which projects into the hoof from behind. 

The comparisons in this part of the work will be between fishes, 
amphibians, reptiles, birds, and mammals, as these represent the great 
type-forms of vertebrates. 


The life in water makes horny cornification very rare. The epider- 
mis of fishes is, therefore, soft. "Pearl organs," however, appear during 
the breeding season in some teleosts. Glands are quite abundant, the 
secretion furnishing the slime on the surface. Some groups of fishes also 
possess poison glands, usually in close relation to the spines of the fins. 
The elasmobranchs have large pterygopodial glands in the "claspers" of 
the males. The purpose of these glands is not known. 

Photophores are some of the most interesting and striking of all 
epidermal organs. They are usually found in elasmobranchs and teleosts 

from the deep seas, where 
sunlight does not penetrate. 
-2 In reality, they are formed 
"^ very much like an eye by the 
.4 cells of the Malpighian layer 
dipping into the corium. 
Here they are cut off from 
-6 their origm, torming a 
_ ^ ^"^^ — deeper glandular layer. The 

outer rounded body is called 

Section through the scale of a Lizard. 1. Peridermal ^hc IcUS, The COrium thcU 

layer. 2. Heavily cornified cells forming the scale. 3. f O r m S a reflecting" laVCr 
Pigment cell. 4. Ordinary cells of horny layer. 5. Inner- . , . ts J ^ 

most Malpighian layer. 6. Dermis. (After Shipley and which m tUm is CUcloSCd by 
MacBride.) . . -^ 

a coat of pigment. 
In the myxinoids many thread-cells in little pockets are located in 
various portions of the skin. Each of these cells contains a long thread 
which is discharged upon stimulation, the threads forming a network 
in which the mucus secreted by the ordinary gland cells is entangled. 
Artificial pearls are made from "essence of pearl," which is formed in 
the fibrous tissue of the corium of some fishes. 

Fig. 395. 


The interesting point about these animals is that during the early 
larval stage the epidermis is often ciliated and two cells in thickness. 
There are numerous mucus and poison glands, sometimes enlargements 
of the neck called "parotid glands." These occur on the anura, and there 
is likewise a gland on the back near the base of the tail. It will be 
remembered that the large lymph spaces under the skin of the frog make 

The Integument 245 

it possible to remove that animal's skin quite readily. As amphibians 
and the lungless salamanders respire largely by the skin, the corium is 
richly supplied with blood vessels, which, at the time of the metamor- 
phosis of anura, penetrate into the epidermis. It is at this time that the 
lungs are not yet functioning, and the gills are being absorbed. The 
stratum corneum is shed periodically, either as a whole, as in urodeles, 
or in patches. The "warts" of toads are partially cornifications of the 
epidermis. A similar hardening of the skin at the ends of the toes results 
in claws. 


All these have horny scales and sometimes bony plates, though some 
of the fossil groups have a naked skin. 

Glands are rare, though some "turtles have scent glands beneath 
the lower jaw along the line between carapace and plastron; snakes and 
crocodilians have them connected with the cloaca, while the latter have 
others, of unknown function, between the first and second rows of plates 
along the back, as well as protrusible musk glands on the lower jaw." 

Comparison of human finger nail (A) 
and hoof of horse (B). 

These latter are not true glands, as they produce no secretion, but cast 
out the living cells. Color changes are not remarkable except in a few 
snakes and lizards. Claws are common on the toes. 

The so-called "femoral pores" on the under surface of the legs of 
lizards are not true glands. They are epidermal structures composed of 
horny cells and possibly have a sexual function. 


The distinguishing characteristic of birds is that they possess 
feathers. Both layers of skin are quite thin. Both scales and feathers 


Comparative Anatomy 

are developed from the epidermis, although there are extremely few 
glands. Some birds, like the ostrich, possess no glands, though a great 
many species have the so-called uropygial gland at the base of the tail 
which pours out an oily secretion for dressing the feathers. In a few 
rasores (scratching birds), there are modified sebaceous glands near the 
ear. The scales on the legs, as well as the claws on the feet and some- 
times on the wings, are often said to be derived from reptilian ancestors. 
Feathers are closely related to scales. There are several kinds of 
feathers, conveniently grouped under three heads : 

Fig. 397. 

Feathers of a pigeon. A, Down feather; B, filoplume; C, quill feather, a.s., 
Aftershaft; i.u., inferior umbilicus; qu., quill or calamus; rch, rhachis or shaft; 
S.U., superior umbilicus; vex., vexillum or vane. (After Borradaile.) 

/, //, ///. — Parts of a feather. /., Four barbs (B) bearing anterior barbules 
(ABB.) and posterior barbules (PBB); II., six barbs (B.) in section showing 
interlocking of barbules; ///., anterior barbule with barbicels (//.) (After Nitzsch.) 

(1) Filoplumes (hairy feathers). 

(2) Plumulae (down feathers). 

(3) Plumae (contour feathers). 

It is the plumae that have the typical form consisting of shaft and 
vane. (Fig. 397.) The base of the shaft is the hollow quill in which 
a small amount of loose pith is found. The shaft or rhachis is solid, and 

The Integument 


a groove runs the length of its lower surface. This is the umbilical 
groove. The vane consists of lateral branches, or barbs, on either side, 
which have, in turn, still smaller side branches called barbules. These 
latter usually have small hooks at their sides and tips. These hooks 
interlock to give firmness and continuity to the whole vane. In down- 
feathers, where hooks are lacking, the barbs arise directly from the end 
of the quill, the barbs do not interlock, and no vane is formed. Hair- 
feathers consist of long slender shafts with a few terminal barbs. 

Archaeopteryx, the oldest known fossil bird, had well developed 
contour feathers. In most birds, feathers are not equally distributed, 

Feather tracts of the pigeon. A, ventral; B, 
dorsal,, alar pteryla or wing tract;, 
cephalic pteryla or head tract;, caudal 
pteryla or tail tract;, crural pteryla; cr.apt, 
cervical apterium or neck-space;, femoral 
pteryla;, humeral pteryla; lat.apt, lateral 
apterium;, spinal pteryla; v. apt, ventral 
apterium;, ventral pteryla, (From Parker 
and Haswell, after Nitzsch.) 

but are gathered in tracts, known as pterylae (Fig. 398), and separated 
by apteria, or featherless regions, where there are but few down or hair 
feathers. These feather-tracts vary, however, in different groups of 
birds, but are used to a considerable extent in classification. 

There is a great similarity in the method in which the integument 
develops in the different type forms we are studying (Fig. 394). For 
example : *'A down-feather begins as a thickening of the corium, push- 
ing the epidermis before it. By continued growth this forms a long, 
finger-like papilla projecting from the skin. The corium extends into 
the outgrowth, carrying blood-vessels with it, Avhile an annular pit, the 
beginning of the feather follicle, forms around the base of the papilla. 
Next, the corium, or pulp of the distal part of the papilla, forms several 
longitudinal ridges which gradually increase in height, growing into the 
epidermis and pressing the Malpighian layer above them against the 
periderm. As a rule, the stratum corneum is divided distally into a num- 
ber of slender rods arising from the base (quill), which at last are only 

248 Comparative Anatomy 

held together by the periderm. Then the pulp retracts, carrying with it 
the Malpighian layer. With the blood-supply removed, the epidermal 
parts dry rapidly, and the periderm ruptures, allowing the rods to sepa- 
rate, forming the down." 

Up to a certain point, contour-feathers are quite like down-feathers 
in their development. 

It is to be remembered that the dorsal and ventral sides of the 
feather were the outside and inside of the stratum corneum of the 
papilla. Scales of lizard skin show extreme similarity in their develop- 
ment to the feather just described (Fig. 395). Many smooth muscle 
fibers act to elevate the feathers in the corium of birds, and there are 
also tactile or sense organs. The colors of feathers depend partly upon 
red, yellow, orange, broAvn, and black pigment deposited in them, but 
the iridescent colors are due to interference spectra. 


Mammals have a relatively thicker skin than other vertebrates (Fig. 
393). There are many glands and considerable hair, except in a few 
orders such as the whales and sirenians. There are likewise horns and 
claws as well as scales, though the latter are not so conspicuous in the 
higher forms. 

The corium is quite thick and is composed of irregular fibers inter- 
laced with muscles, blood vessels, etc. Its outer surface often forms 
papillae or ridges, especially on the palms and soles. These ridges carry 
the epidermis with them. Several strata may usually be recognized 
under the epidermis, namely : a thick Malpighian layer at the base, then 
a thin stratum lucidum in which distinct cells cannot be recognized, and 
the stratum corneum on the outside. One or more other layers may be 
present. A cell must pass through all of these layers before it is worn 
from the surface of the skin. 


It is important that the histological structure of a hair (Figs. 393, 
394) be compared with that of a feather already described. 

Scales are found in many orders, usually best developed on the tail 
and feet. They are rounded, quadrangular, or hexagonal, the square 
scales being arranged in rings around the part, the others in groups of 
five known as quincunx. These latter are closely similar to the scales 
of reptiles. It seems, from recent investigations, that there is a close 
relation between scales and hair, since in mammals with scales, hairs are 
usually arranged in groups of three or five behind each scale; and in 
those without scales, the hairs are also grouped in the same manner. In 
the early embryo, the hairs are arranged in longitudinal rows so that 
grouping seems to come later. 

The Integument 


These are of various kinds and types. The structural shapes and 
forms into which they may be grouped have already been studied in the 
frog- and should be recalled, but we must also think of five divisions or 
groups, classified not according to structure, but according to function. 
Thus we have the following grouping: 

(1) Sweat (tubular in shape), extending from the Malpighian layer 
down through the corium where they are coiled. 

Schematic arrangement o£ varying types of mammary glands. 1, Echidna, 
primitive type; 2, Halmaturus (a genus of Kangaroo) forming pouch in lactation; 
3, Didelphys, forming of nipple before lactation; 4, Same during lactation (quite like 
man); 5, Mammilary pouch in cow embryo; 6, in adult cow. (After Max Weber.) 

(2) Sebaceous (acinous in shape), connected with each hair (Fig. 

(3) Mammary (modified tubular glands) which produce milk 
(Fig. 399). 

(4) Tarsal or Meibomian (modified sebaceous glands), in eyelid, 
producing oil to keep tears from overflowing (Fig. 400). 

(5) Anal (acinous in form), commonly scent glands, secreting a 
substance either for sexual attraction or for protection (Fig. 400). 

Glands are often also divided according to the method by which 
they furnish their secretions. First, necrobiotic glands, which burst 
when liberating their fluids. The individual gland is then destroyed. 
And, second, vitally secretory glands, the secretions of which are poured 
through the walls of the gland while the gland itself remains functional 
for an indefinite period. In fact, this physiological distinction is often 
used to determine homologies when other methods cannot be used. 

Each animal class seems to develop integumental glands in its own 
peculiar way. No definite and continuous history of gland development 
may be found throughout the various groups. Those animals living 
in the water, such as fish and amphibians, have glands that secrete pro- 
tective substances which are often poisonous. The Sauropsida seldom 
have any integumental glands at all, and snakes have characteristic 
cloacal glands secreting a particularly nauseating substance. Certain 
turtles have so-called musk-glands, probably for sexuaF attraction. In 
some lizards there is a row of so-called glands (really femoral pores), 


Comparative Anatomy 

along the inner portion of the femora, that secrete a substance at breed- 
ing time which hardens into short spines or teeth. In birds there are 
only the uropygial glands in the caudal region, which furnish an oil 
for the feathers. 

In mammals there are many and varying glands in the skin, but 
they may all be placed into two groups (Fig. 400), namely, the sweat- 
glands, which are vitally secretory and tubular, and the acinous glands, 
many of which are lobed and necrobiotic, although both originally arise 
in connection with the hair. 

The secretion from the sweat glands is usually thin and watery. 


i>C J/7C/ S 

Fig. 400. 

A, Sweat gland; B, Acinous gland. Complete gland and cross section, 
cross section is cut at the level of the arrow. (Compare with Fig. 393.) 


although it may vary from this to a thick viscous pinkish fluid, the 
so-called "blood-sweat" of the hippopotamus. 

The sweat glands may be found almost anywhere on the entire body 
or they may be localized. Localization takes place most frequently in 
the paws or on the palms of the hand and soles of the foot. Here they 
serve to assist in grasping a given object more solidly. 

A modification of these glands also furnishes the oily secretion of 
the ear. 

It probably has been observed that in hot weather horses sweat 
quite profusely while dogs do not. This is due to the fact that horses 
have sweat glands in the skin while dogs have not, so that dogs can only 
obtain the same relief that other mammals obtain, during such weather, 
by opening their mouths and panting, as it is only in this way that the 
constantly accumulating moisture finds its way to a surface where evap- 
oration then brings about a cooling. Muzzles should, therefore, always 
permit the opening of the mouth. 

There are some races of men, such as the Fuegians, who likewise 
have few sweat glands. 

The acinous glands furnish an oily secretion, apparently for the 

The Integument 251 

original purpose of lubricating the hair, regardless of how far removed 
from this function such glands may ultimately come to be. These glands 
are called sebaceous. The tarsal, or meibomian, glands of the eyelids 
are practically hypertrophied sebaceous glands of the eyelashes. These 
meibomian glands pour out an oily secretion which lubricates the edge 
of the lids and prevents tears from overflowing. There are modified 
sebaceous glands in the various orifices of the body such as in the lips 
and about the anus. 

Then there are groups of glands which are localized for quite specific 
functions, such as the anal-sacs of the skunk, which secrete a protective 
substance, and the sexually attractive glands, such as those of musk or 
civet. Musk is often used in the manufacture of perfumes. 

Glands usually open as an elevation at a single place, known. as a 
glandular area. The milk-glands of mammals are typical examples, but 
there are cases where there is a sinking of the area so that instead of 
the young taking a nipple in their mouths, the lips of the sunken area 
fit closely about the nose of the young and thus prevent the secretion 
from being lost. Such is the case in Echidna (Figs. 383, 399). In the 
opossum the nipple is really a sac like that in Echidna, but turned inside 

It is a common observation that in many of the domesticated animals 
there is a row of nipples extending from axilla to the groin. In the 
embryo of many placental animals there is an entire ridge along which 
the mammary glands are to appear. In a short time there are suppres- 
sions at regular intervals, which leave protruding nipples. These nipples 
in turn become reduced and eventually become actual depressions. 

The varying position of the nipples in different groups of animals 
is due to the retention of some of the nipples in a particular region and 
the suppression of the remaining ones along this lateral ridge, which, 
as mentioned, extends from axilla to groin. 

It is of interest that the aquatic Sirenia (Fig. 390) have pectoral 
mammary glands. They bear but a single young at a time and nurse 
their offspring by standing erect in the water while clasping the young 
in their flippers. It is supposed that many of the 'mermaid stories had 
their origin from an observation of this animal nursing its young. 

It is by no means uncommon to find animals (including man) having 
a peculiar arrangement of nipples on their bodies. Supernumerary nip- 
ples are termed hyperthelism, and supernumerary mammae are termed 
hypermastism. These supernumerary developments sometimes occur on 
the thigh and other parts of the body. They are considered displace- 
ments and not reversions if they occur in out-of-the-ordinary regions, 
and reversions if they occur in regions where they normally develop 

While rudimentary nipples occur in the male of placental mammals. 

252 Comparative Anatomy 

and may even prove to be functional in some instances, monotromes and 
marsupial males do not develop them at all. 


As the dog-fish has w^hat is called the indifferent type of an exoskel- 
eton, it is this animal which forms the classic example for a preliminary 
study. Here we find imbricated rows of pointed scales (which merely 
means that one row of scales covers the intervals of the next row). 
(Fig. 401.) The scales of other fishes, as well as of reptiles, and even 
the feather papillae of birds, and the hair of mammals, are all arranged 
in a similar manner. 

The scales of the dog-fish are said to be placoid (Fig. 401), which 
means that each has an approximately flat base from which a sharp- 
pointed cusp arises. This cusp is inclined in the direction of the free 
edge of the scale. When the scale is in place, the inclination is toward 
the posterior portion. 

The scale itself consists of a core of dentine which is overlaid with 
enamel. In fact, the cusp is almost all enamel. The papilla from which 
nourishment comes to the scale lies beneath. In the embryo the scale 
forms between epidermis and corium, the dentine arising from the 
corium, and the enamel from the epidermis. 

In the selachians there are several rows of pointed teeth arranged 
quite like the scales on the surface. These develop from the same layers 
and in the same manner as the scales, and consist of a similar structure, 
so they are assumed to be merely placoid scales modified by different 

All higher vertebrates inherit teeth. In birds and turtles they are 
supposed to have been secondarily lost. 

Ganoids (Fig. 368) develop their scales (Fig. 401) from the corium 
alone, the epidermis playing no part. Consequently the ganoid scale is 
all dentine. Ganoid scales are shiny, which is the very meaning of the 
term "ganoid." They are usually rhomboidal in shape and do not pos- 
sess a cusp. In the sturgeons (Fig. 368), the scales consolidate into 
large bony shields called scutes. The former mailed or armoured fishes 
merely carried this consolidation to great extremes, and the plates were 
continuous. In the sturgeons the plates are not continuous but are 
placed in rows along the back and sides so that there are large areas 

It is important to note at this point that in all ganoids (Polypterus, 
sturgeons, paddle-fishes, gar-pikes, and bow-fins, Fig. 368), the plates 
or scutes cover the entire head. The coming together of the edges forms 
sutures while the structures lying between the sutures are commonly 
called the dermal bones of the skull. 

Frontals. parietals, maxillaries, and squamosals are found in all 

The Integument 


higher groups, though the opercular and rostral series disappear entirely. 
Most of the orbital series also disappear, with the exception of the 
lacrimal. Then, too, there are the dermal bones of the mouth cavity, 
such as vomers, palatines, and parabasal, which are supposed to have 

l-h^ ^' ^. 

Fig. 401. 

Placoid scales. A, A portion of the skin of the dogfish as seen under a hand 
lens; B, a single scale removed from the skin; C, the same in section (diagram- 
matic). b.. Base of the scale; c, the same in section; d., dentine; e., enamel; 
p., pulp cavity. D, Part of the tail of a dogfish seen from the left side, with a 
piece of the skin removed. l.L, Tube of the lateral line; myc, myocommata or 
septa of connective tissue; mym., myomeres. (After Borradaile.) 

E, ctenoid; F, ganoid; and G, cycloid scales. (From the Cambridge Natural 
History; E, F, after Giinther; G, after Parker and Haswell.) 

retained the original character, inasmuch as teeth often form on and in 
these bones. 

In the higher forms, the dermal bones, however, arise from various 
centers of ossification in the cutaneous mesenchyme, and while this 
difference has been explained as a curtailing of the previous race history, 
it is quite likely that there is little difference between dermal and 
cartilaginous bone formation in the highest mammalian forms, the 
dermal being merely more "stretched out" portions, as will be learned 
when the endoskeleton is studied. 

Scales in the teleosts, although often rhomboid when quite young, 
become circular later and are then called cycloid. Ctenoid scales are 
quite similar to cycloid except that they are set in diagonal rows in 
pockets of the dermis with their free edges overlapping. (Fig. 401.) 

254 Comparative A.n atomy 

Amphibians do not have scales and hard exoskeletons, although 
there are extinct forms in v^hich the body was covered v^ith them. 

In reptiles, the scales arise only from the epidermis and are, there- 
fore, composed of horn or keratin. There is no trace of bone in them. 
The corium, hov^ever, furnishes the nourishment to these keratin scales, 
although it does not furnish any of the hard parts. There is no definite 
knowledge as to the relationship between the scales of reptiles and 
those of fishes. 

Reptiles also have other integumental structures beside the keratin 
scales, namely: spines, combs, and claws; all, however, also made of 

The birds are structurally and developmentally quite like the rep- 
tiles in that they possess feathers which are homologous to the reptile 
scales, and in having their beaks and claws composed of keratin. There 
have been toothed-birds in the past, and it is said that tooth-germs have 
even been found in the embryonic jaws of some of our modern birds. 

In mammals, the tiny scales covering the body are seen as definite 
hard structures, mainly on the claws, tails, and sometimes on the backs 
of such animals as the armadillos (Fig. 387). They are always only 
epidermal in origin. In the armadillos, the corium secondarily supplies 
the hardening portions so that the covering of the animals becomes very 
thick, hard, and osseous. 

It is assumed, very often, that formerly all mammals were covered 
by hard scales because the hair arrangement of mammals is quite like 
that of the scales. For example, on the tail of a rat, the scattered hairs 
will appear among the scales in a very definite relationship, namely, a 
group of three hairs (one medial and two lateral) will project beneath 
the margin of each. The median hair is the longer and stouter. In 
addition to this, there are similar arrangements of hairs in groups of 
three even upon areas not definitely associated with scales. The hairs, 
however, are arranged in an imbricated series like scales. Even where 
the hair is very thick, and forms a heavy fur, this arrangement can often 
be made out. 

As scales in their simplest form are tiny elevations, the pads on 
mammalian feet are often used to illustrate the arrangement and transi- 
tion of scales in different mammalian groups. 

These pads are usually eleven in number, five for the tips of the 
digits, four for the distal margins of palm and sole beneath the inter- 
digital intervals, and two for the wrist or ankle. 

The scale rudiments are arranged in rows upon these pads. They 
fuse to form "friction ridges," so-called because they prevent the animal 
from slipping (Fig. 402). These friction ridges are always arranged at 

The Integument 


Fig. 402. 

Ventral view of the palm of the hand of an 
insectivore and of a primate to show correspondence 
between relief and arrangement of friction ridges. 
A, Crocidura caerulea (shrew-mouse). Forepaw 
showing walking-pads enclosed by triangular folds of 
skin. B, Macacus sp? (Old World monkey). Hand, 
covered by friction ridges, the arrangement of which 
corresponds to the relief of A. The pads are rep- 
resented by concentric circles, and the trianglar folds 
by triradii. These latter features are here designated 
by heavy lines, although in the animal they are no 
more conspicuous than the others. (From Wilder after 
Miss Whipple.) 

right angles to the direction in 
which there is considerable 
tendency -to slip. In the arbo- 
real types of mammals, such as 
the lemurs and monkeys, the 
scale rudiments are arranged 
in concentric circles, as in such 
animals there is a tendency to 
slip in any and all directions. 
The ridges form only on the 
actual contact surfaces. 

While structure always de- 
termines function, yet in integ- 
umental studies we have found 
that function very decidedly 
modifies the various structures, 
and later, we shall see that such 
modification is not confined to 
integument alone. 

Now, to be truly scientific, 
means to retain an open mind 
to all truth wherever and whenever found. But our prejudices and 
wishes all too often influence us as readily toward a too conservative as 
toward a too radical point of view. We must face the facts as they are, 
pleasant or unpleasant, but we must not forget that many different in- 
terpretations can be drawn from the self-same facts. An example of 
this is brought home at this very point. 

There is no question about the facts so far presented, which anyone 
can demonstrate for himself. The question that presents itself is simply 
this: Does it follow that because a bird has all the characteristics of a 
reptile, plus some additional features, that, therefore, it had reptile 

If one accept the so-called Haeckelian law of biogenesis, that each 
individual in the embryonic stage passes through the adult stages of the 
race to which it belongs, then such a conclusion is valid; but, if we 
remember that all this so-called law means is that all forms pass through 
similar stages, the higher forms then continuing, while the lower ones 
remain stationary, another interpretation is still more valid. And our 
difficulty is by no means lessened when we remember that biologists at 
large are agreed that acquired characteristics are not transmitted. What, 
then, becomes of even a reasonable explanation of how any modifications 
can be carried on from parent to offspring? 

Still further, we have seen from Professor de Vries' work that all 
newly appearing structures may be but the return of some recessive 

256 Comparative Anatomy 

characters which have long lain dormant, while in the so-called rudi- 
mentary structures there is always the alternative of considering such 
structure an overgrowth or a hypertrophy of some smaller organ valuable 
at some time in embryonic life; or, it may even be a true remnant of a 
structure no longer needed by modern methods of life, modern foods and 
modern environment. Or, still a third alternative suggested by Professor 
Bateson, that just as a complex structure is the more complex, the 
smaller and simpler it can be made to appear, so the original fertilized 
egg-cell, from which an entire vertebrate develops, is much more complex 
than the finally completed body, because the single cell had all the possi- 
bilities of the complete body within its tiny self, and consequently, we 
are always really losing something as development proceeds. 

What is meant by a normal development of a cell into what it is 
later to become, is simply, that commonly, certain obstacles are removed 
by which these possibilities can come forth. If then, either environment 
(external or internal), food, atmosphere, position, injury, or chemical 
stimulus removes certain factors which hold back growth, any such 
possible factor already present in the cell may come forth ; but its pos- 
sibility must have been already present in the primitive cell. 

This is well shown by the fact that normally the skin finishes growth 
at a certain time, but if a portion of skin is torn, the injury stimulates 
the connective-tissue cells which then divide and fill the wound with 
scar-tissue, that is, the original injury removes an obstacle to such con- 
nective-tissue-cell's growth. What particular factors, then, can be said 
to explain modifications? We do not know. 

It is the province of science to press a problem further and further 
back and thus raise more problems. There is, and can be, nothing 
absolute about any scientific interpretation. 

The student, as does the average man, wants something definite, 
something he can be sure of; but this is just what he cannot find in any 
biological study ; and, unless he can appreciate this and still love science 
— science is not for him. 

If he should nevertheless go into a scientific field such as medicine 
or dentistry, he will be a practitioner who will ever seek and follow the 
opinions of the least scientific and least trustworthy men, simply because 
these speak with definiteness and absoluteness, albeit, likewise with 



BY the term skeleton we mean all hard parts used for support and 
protection outside of what has already been termed the integument. 
The skeleton develops only from mesenchyme. It will be recalled 
that after the mesoderm has divided into a somatic and a splanchnic 
layer, these two layers together are called mesothelium to distinguish 
them from the mesenchyme. The latter, while also lying in the seg- 
mentation cavity, develops as separate cells from both the mesothelium 
and the entoderm. Some even believe that ectoderm has a part in its 

When bone forms from 
cartilage, the lime salts may 
be laid down on the inner 
portion of the perichon- 
drium and from there 
^^^' ^^^' invade the cartilage. This 

Diagram to show growth of bone. A, animal recently • 11 j 'r .- 1 

fed madder which causes a layer of bone (black) to be IS Called OSSltlCatlOn by 

colored by the dye; B, no madder fed for a time, when a api-r»^Vir»nr1rncfncic 

deposit of colorless bone on outside of colored layer is eCtOCnonurO'StOSlS. 
formed; C, later the outer layer becomes thickened and the 
inner layer is absorbed. BonCS may form from 

cartilage of the osteoblasts or, forming from the more interior cells and 
then, with this group of osteoblasts as an ossification center, ossification 
extends in all directions. This latter method is known as entochondros- 

Often the long bones increase by the formation of smaller bones 
which then become attached to the ends of the long bones. Such joining 
is called an epiphysis. 

If madder is fed to an animal, the actual bone formation is colored. 
This makes it possible to see just how the new bone is formed. The new 
bone is laid down outside of that already grown, and with such growth 
the "marrow cavity" becomes larger by a resorption of the bone which 
has already formed. The osteoblasts are laid down in between the newly 
forming layers of bone (Fig. 403). 


We have already seen, in our study of the embryology of the frog 
and chick, how the centra of the vertebrae are formed around the noto- 
chord and that possibly some parts of the chorda remain as the inter- 
vertebral discs. Here we are to study and compare the adult form in 
the various groups. 

The most complete vertebrae may be found in the tails of some of 
the lower vertebrates. Figure 404 shows a comparison of several types. 


Comparative Anatomy 

It will be noticed that dorsally there is a neural arch, while ventrally 
a similar outgrowth from the centrum is known as the haemal arch, 
while the pointed end in each case is known as a spine. 

The haemal arch is quite incomplete or even entirely absent in the 
regions anterior to the tail. 

In the higher vertebrates (Fig. 404) there are articular processes, 

Fig. 404. 

/, A and S. Diagram of a vertebra of a bony fish. A, caudal; B, trunk; C 
amphicoelous; D procoelous; E, opisthocoelous; F, amphiplatyan vertebrae. The 
head is supposed to he at the left, c, centrum or body of vertebra; ch. notochord; 
h.a., haemal arch; h.c, haemal canal; h.s., haemal spine; h.z., haemal zygapophysis 

The Endoskfxeton 259 

called zygapophyses, both on the anterior and posterior sides of each 
vertebra, and usually transverse processes extending laterally in the 
planes of the original divisions between the muscles. 

Where true ribs occur, there are two additional transverse processes 
to which these attach. 

The centrum, where it meets with the intervertebral disc, has four 
distinct forms (i^ig. 404). 

If the face of the centra at each end, where it is to meet with the 
intervertebral disc of the centra lying immediately anterior and imme- 
diately posterior to it (as in hshes), is hollow at both ends, it is called 
amphicoelous (Fig. 404). If one end is like a ball, namely, convex, and 
the other concave, so that the ball-like portion can lit into it, the condi- 
tion is known as procoelous if the socket lies on its anterior surface, and 
opisthocoelous if on the posterior surface, while if the ends of the centra 
are flat, as they usually are in mammals, such a condition is known as 

The arches of the vertebrae form first (Fig. 352), and the centra 
later, and the sclerotome divides into a caudal and cranial half which 
thus makes possible an advantageous condition to the animal in permit- 
ting interaction of skeleton and muscles (Fig. 305). 

It must be remembered, however, in this connection, that in some 
animals normally, and in others abnormally, the two halves of the 
sclerotome may unite (as in some fishes), and thus not have this inter- 
play of muscles; or two neural arches may form by the rudiment which 
normally becomes one arch, dividing as does the sclerotome, and thus 
produce a greater quantity of vertebrae than usual. And, not only may 
this happen to the neural arches, but also to the centra. In fact, almost 
any variation in the spinal column may be accounted for by an embryo- 
logical condition remaining in the adult form. 

or articulating facet; m.b., intermuscular bone; n.a., neural arch; n.c, neural canal; 
n.s., neural spine; n.z., neural zygapophysis; r, rib. 

//. Composition of vertebrae of Reptiles, illustrated by the first and second 
cervical vertebrae. (1) Atlas (first cercival) and axis (second) vertebrae of 
Crocodile. (2) Atlas and axis of Metriorhynchus, a Jurassic Crocodile. (3) 
Analysis of the first two cervical vertebrae of a Crocodile. 2, second basiventral 
complex or "intercentrum" continued upwards into the meniscus or intervertebral 
pad. (4) Diagram of the fundamental composition of a Reptilian vertebra or 
other amniotic, gastrocentrous vertebra. (5) The first three cervical vertebrae 
of Sphenodon. (6) Trunk-vertebrae of Eryops, a Permian Proreptile, typically 
temnospondylous. cp, articular facet of the capitulum of a rib. (7) The complete 
atlas of an adult Trionyx hurum. The second basiventral (intercentrum) is 
attached to the posterior end of the first centrum which, not being fused witli the 
second centrum, is not yet an odontoid process. (8) The complete atlas of an 
adult Trionyx gangeticus, still typically temnospondylous. (9) The first and second 
cervical vertebrae of an adult Platemys. (10) The complete atlas of a Chelys 
fimbriata. Az, Anterior zygapophysis; B.D, basidorsai; B-V, basiventral; C^, C.,, C,, 
first, second and third centra, formed by the interventralia; Cp^, Cp'^, articular 
facets of the capitular portions of the first and second ribs; /, V, Inter- 
ventral; N^yN^ N^, first, second and third neural arch; formed by the basidorsalia 
(B.D); Od, odontoid process (which is the first centrum); Ps-, posterior zygapophysis; 
R^, /?2» ribs; Sp, detached spinous process of the first neural arch t^, t.„ tubercular 
attachments of the first and second ribs; 1, 2, 3, 4, "intercentra" (which are the 
basiventrals) ; I, II, III, position of the exit of the first, second and third spinal 

III. TrutTk vertebrae of a tropical Skate*. /;, haemal process; i, intercalary 
plate; n, neural process; r, rib; s, spinous process. (//, After Gadow; ///, from 
Kingsley after Dumeril.) 

260 Comparative Anatomy 

As the ventral nerve root usually penetrates the caudal division of 
the halved sclerotome, and the dorsal root passes betv^een the two 
divisions of each sclerotome but penetrates the cranial portion, one can 
tell in the adult, from follov^ing these nerve roots, which of the adult 
structures come from cranial and which from caudal halves. 

The different shaped ends of the centra, which have already been 
mentioned, are brought about after the vertebrae are quite definitely 
formed. The centra with their arches are in a quite definite position 
and the centra cannot, therefore, grow any more except at the ends. 
These may have more substance laid down in the intervertebral regions, 
however, and thus ultimately come to be amphicoelous, procoelous, 
opisthocoelous, or amphiplatyan. 


The regions of the spinal column are: 

(1) Cervical. The neck region, either without ribs of any kind or 
the ribs are smaller than in the other regions. 

(2) Thoracic. These have distinct ribs attached. 

(3) Lumbar. Following the thoracic, and without ribs. 

(4) Sacral. This region includes one or more vertebrae with which 
the pelvic girdle is connected. 

(5) Caudal. The tail-portion immediately following the sacrum. 
These divisions are quite distinct in the higher vertebrates, but in 

the lower, any and all combinations may form, so that the ribs may 
extend almost the entire length of the spinal column. In such cases all 
vertebrae having ribs are called dorsal. 

In fishes, snakes, and whales, the sacral region cannot be distin- 
guished ; and in fishes, the dorsal and cervical vertebrae are quite 
indistinguishable. In this latter case there are, therefore, only trunk or 
abdominal vertebrae, and caudal vertebra, the line being drawn where 
the haemal arches begin to have ribs attached. 

The first cervical vertebra to which the skull is articulated is called 
the atlas in all higher vertebrata, while the second cervical vertebra, at 
least in the amniotes, is called the axis or epistropheus (Fig. 404). 

In mammals the atlas can always be distinguished by the two 
anterior articulating surfaces for the two condyles of the skull, and the 
axis, by the tooth-like projection known as the odontoid process, on 
which the atlas turns. 

It is interesting to note that embryologically, this tooth-like process 
develops from the atlas, but then separates and later becomes attached 
to the next succeeding vertebra. • 

In a few reptiles there is a so-called proatlas, consisting of one or 
two plates lying between the atlas and the skull. It is not known just 
what relationship this bears to the other vertebrae. 

In fin-bearing animals, if the spinal column runs to the end of the 

The Endoskeleton 


body in a straight line (Fig. 405), the caudal fins are known . as 
diphycercal, a condition found in the young of all fishes and in adult 
cyclostomes, dipnoans, and crossopterygians. 

If, as in the elasmobranchs and ganoids, the tail axis bends abruptly 
upward at the end, but retains the dorsal fin-part and a portion of the 
ventral region, it forms what is called a heterocercal tail, while if there 
is the same upward bend of the spinal column but the ventral and dor- 
sal fin-portions of the tail become alike as to size and shape, the tail is 

Fig. 405. 

Diagrams of the principal forms of tails in fishes. A, proto- 
cercal fin (as in Polypterus) ; B, heterocercal (as in sharks) ; C, homo- 
cereal (as in most teleosts); D, homocercal (as in Amia). (After 

said to be homocercal. Homocercal tails are brought about by the neural 
arches becoming smaller and the haemal arches becoming larger. 


Bone either forms in cartilage or membrane, and it is quite common 
to hear biologists speak of cartilaginous and membranous bone. How- 
ever, recent investigations lead us to believe that the so-called membrane 
is nothing more or less than cartilage drawn out very thin in those parts 
where the greatest pressure is produced. This can be understood the 
better if Fig. 406 be carefully studied. It will be noticed that all the 
superior and inferior boundaries are membranous, for here there is 
nothing to prevent a considerable extension of growth, while in the 
innermost portions, where pressure comes from practically all sides, it 
is cartilage. 

Babies have a very soft spot on top of and close to the center of 
the head for about one and one-half years after birth. Places such as 
these are called fontanelles. These fontanelles are found during the 
embryonic period at all spots in the skull where several points of ossifi- 
cation come together. Ossification begins at many points, each center of 
ossification extending and growing toward each other. The fontanelle 


Comparative Anatomy 

is the unossified spot that constantly becomes smaller until ossification 
is complete. 

Professor Eben J. Carey has recently shown that, contrary to the 
usually accepted idea that bone grows simply because there is an inner 
something which makes it assume definite forms, it is the stress and 
pull and pressure of its location which determines its shape, size, rapidity 
of growth, and even its joints. 

The reason this has not been understood heretofore is that former 
experimenters took only sections from the growing bone itself for their 
study. Professor Carey, however, has taken the complete embryological 
structure, including all muscles and related portions, which might throw 
light upon the pull and pressure which affects such bone during its 
growing period. 

Observing the ossification centers in the skull will throw light on 
this subject (Fig. 406). There are many such centers, and they are 
always found at exactly those points where there is an especial stress or 
pressure. At these points it may be that sharper bends in the blood 
vessels cause a slowing of the blood stream, which slowing in turn causes 
lime salts to be laid down at the angles to a much greater extent than 
where the blood stream can rush past more swiftly. Then, with each 




Fie. 406. 

A diagram of the skull bones of a mammal, the membrane 
bones shaded. B.O., Basioccipital; E.O., exoccipital; C, condyle; 
S.O., supraoccipital; Par., parietal; Fr., frontal; Na, nasal; Pmx., 
premaxilla; M.E., mesetlmoid; L., lachrymal; T2t.._ turhi al; P.S., 
presphenoid; O.S., orbitosphenoid ; A.S., alisphenoid; B.S., basis- 
phenoid; S.Q., squamosal; P., periotic; T.. tympanic; P'.. palatine; 
Pt., pterygoid; Mx., maxilla; Ju., jugal; T.H., tympanohyal; S.H., 
stylohyal; E.H., epihyal; C.H., ceratohyal; B.H., basihyal; Th.H., 
thyrohyal; vomer; MN., mandible. (From Borradaile, modified 
from Flower and Weber.) 

succeeding deposit of such a hardening substance, a still greater number 
of blood capillaries is affected so that more lime is laid down, and so 
on, until all of the capillaries have been more or less obliterated and the 
entire cartilage, or membrane, has become ossified. 

Beginning with the axial skeleton, the skull becomes our first object 
of attention. The cranium is that part of the skull which encloses the 
brain, together with those bony parts forming the eye-socket, the ear. 

The Endoskeleton 363 

and the nose. The more caudal portion of the skull, which is directly 
connected with the cephalic end of the digestive tract, is called the 
visceral skeleton. 

That portion of the skull which is cartilaginous is known as the 
chondrocranium while the membranous portion is called a membrano- 

On each side of the notochord (which in the embryo extends as far 
forward as the infundibulum of the brain) a horizontal plate of cartilage 
is formed. These plates are known as parachordal plates (Figs. 310, 
353). These extend laterally to the ears, forward to the end of the 
notochord, and backward to the exit of the tenth nerve. The otic capsule 
(cartilaginous) then grows about each internal ear and joins the para- 
chordals. This forms a sort of trough in which the most caudal portion 
of the brain lies. The floor of this trough is known as the basilar plate, 
being formed of the parachordals and notochord as a floor, while the 
sense capsules constitute the sides. 

From this basilar plate two cartilages pass forward on each side, 
forming a similar trough for the anterior part of the brain. According 
to Professor Kingsley, the lower of these, called the trabeculae cranii, 
"join the anterior margin of the basal plate, while the dorsal bars, the 
alae temporales or alisphenoid cartilages, are eventually connected with 
the anterior wall of the otic capsules. In most vertebrates the trabecu- 
lae and alisphenoids develop as a continuum, but in some elasmobranchs 
they are at first distinct. The two trabeculae unite in front to form a 
median ethmoid plate beneath the olfactory lobes of the brain, beyond 
which they diverge as two horns, the cornua trabeculae, ventral to the 
nasal organs. The floor of the trough in front of the ears is formed by 
the ethmoid plate anteriorly, while behind, it is usually of membrane. 
In the elasmobranchs, cartilage gradually extends from one trabecula 
to the other, closing last below the infundibulum and hypophysis, 
these lying for a time in an opening (fenestra, later fossa hypophyseos), 
and after the closure, in a pocket in the floor of the chondrocranium, one 
of the cranial landmarks, the sella turcica." 

*Tn the more primitive vertebrates, the trough is converted into a 
tube around the brain by the extension of cartilages between the 
alisphenoid cartilages and the otic capsules of the two sides dorsal to the 
brain. This roof, or tegmen cranii, is usually incomplete, having one or 
more gaps or fontanelles, closed only by membrane. In the higher ver- 
tebrates the cartilage roof is at most restricted to a mere arch, the synotic 
tectum, between the otic capsules of the two sides. 

"Later a pair of nasal capsules develop around the olfactory ors^ans. 
These are usually fenestrated and become united to the cornua, alisohe- 
noids, and ethmoid plate. In a similar way a sclera (sclerotic coat) forms 
around each eye, but since the eye must move, this sense capsule 
never unites with the rest of the cranium. Behind the otic capsules 


Comparative Anatomy 

a varying number of (four in some sharks and most teleosts ; in others 
three; in amphibia two), occipital vertebrae are developed, which later 
fuse with the rest of the chondrocranium. They alternate with myotomes 
and nerves in this region as do the vertebrae of the vertebral column. 

"The cartilaginous visceral skeleton arises in the pharyngeal region 
which is weakened by the presence of the gill clefts. It consists of a 
series of pairs of bars, the visceral arches lying in the septa between the 

otic capsule 



olfactory capsule 

n«^ arch intercalary arch 


jnandibular (first) 

gill arch 






or copula 

hyoid (second) 
gill arch. 

phi op.vii. 

ip. n.a. n. 

i:„a /. ,/. 


Fig. 407. 

I. Diagram of the chondrocranium, vertebral column, and gill arches of an 
elasmobranch to show particularly the parts and relations of the seven gill arches. 
(Hyman's modification of Vialleton.) 

II. Skull and part of the backbone of a dogfish, seen from the right side. The 
skeleton of the visceral arches has been pulled a little downwards, au.c, Auditory 
capsule; b.b., basibranchial cartilage; h.h., basihyal cartilage; c, centrum; cer.h., 
ceratohyal cartilage; cer.b., ceratobranchial cartilages; d.r, v.r., forarnina for the 
dorsal and ventral roots of a spinal nerve; e.m,., extrabranchial cartilages; e.c.f-, 
external carotid foramen; ep.b., epibranchial cartila.ges; e.L, ethmopalatine liga- 
ment; gr., groove for vein which connects orbital and anterior cardinal sinuses; 
g.r., gill rays; hy.a., foramen for hyoidean artery; hymd., hyomandibular cartilage; 
i.o.c, interorbital canal; i.p., intercalary plate; M.c, Meckel's cartilage; I.e., 
labial cartilages; nas.c, nasal capsule; n.a., neural arch; n.sp., neural spine; o.n.f., 
orbitonasal foramen; op.V., op. VI I., foramina for ophthalmic branches of fifth and 
seventh nerves; op'., foramen through which combined ophthalmic nerves pass from 
the orbit to the snout; op.g., grooves for op.V., VII.; orb., orbit; p.sp.L, postspiracular 
ligament; pal.b., palatine bar; ph.b., pbaryngobranchial cartilages; r., rib; rost., 
rostrum; tr., ventrilateral (so-called transverse) process. (After Borradaile.) 

The Endoskeleton 


clefts, the bars of a pair being connected below the pharynx. Each 
bar, at first, is a continuous structure, but to allow for changes of size in 
the pharynx, each becomes divided into separate parts, while the arches 
become connected in the mid-ventral line by unpaired elements, the 
copulae (Fig. 407). The two anterior arches are specialized and have 
received special names, the first being the mandibular, the second the 
hyoid arch, the others, in the region of the functional gills, being called 
collectively gill, or branchial arches. The number of these last varies 

.Meckel's cartilage 
Hyuid (lesser horn) 

^ A, B, C, Bones of early human skull to show their compound nature. A, 
occiptal bone at birth showing the five elements of which it is composed. B, 
sphenoid bone in an embryo of four months. C, temporal bone at birth, showing its 
three components. Cartilage represented in black. (Redrawn from Sappey and 

D, Diagram of skull of new born child. White areas represent bones of 
intramembranous origin; dotted areas represent bones (not derived from branchial 
arches) of intracartilaginous origin ; black areas represent derivatives of branchial 
arches. (Combined from McMurrich and Kollmann.) 


Comparative Anatomy 

with the number of gill clefts. There are seven gill arches in the primi- 
tive sharks, a smaller number in the higher groups, in which, with the 
loss of branchial respiration, their form and functions may be altered. 
At first all are clearly in the head region, but by the unequal growth of 
cranium and pharynx the gill arches are carried back some distance 
behind the head. All of the arches are cartilaginous at first." 

The mandibular arch lies in the region of the fifth nerve, behind 
the mouth and between it and the first visceral cleft — the cleft that 
becomes the spiracle or Eustachian tube. "The arch is divided into 
dorsal and ventral halves, known respectively as the ptery go quadrate 
(palatoquadrate) and MeckeUan cartilages. In the elasmobranch and 


X-^j% \ 


ntj „ --^ ' poo ^ 


Fig. 409. 

Side view of a mackerel, ar, articulare; as, alisphenoid; bo, 
basioccipital ; d, dentary; enp, entopteryoid; eo, exoccipital; ep, 
ectopterygoid; es, extrascapular; epo, epiotic; /, frontal; io, inter- 
opercular; eth, ethmoid; I, lacrimal; mx, maxillary; mxp, metaptery- 
goid; n, nasal; op, opercular; p, parietal; pe, petrosal; pf, post- 
frontal; pi., palatine; pm, premaxillary; po, preoperculum; poo, 
postorbital; prf, prefrontal; ps, parasphenoid; g, quadrate; sbo, 
suborbital; so, supraoccipital; sop, suboperculum; spo, sphenotic; 
sg, squamosal; ssc, suprascapular; sy, symplectic. (From 
Kingsley after Allis.) 

chondrosteous ganoids the ptery goquadrate forms the upper jaw, lying 
parallel to and joined to the cranium by ligaments or in chimaeroids by 
continuous growth. With the appearance of bones a new upper jaw is 
formed, as described below, and the pterygoquadrate becomes more or 
less reduced, and ossifies as two or more bones with greatly modified 
functions. Meckel's cartilage is the lower jaw of the lower vertebrates, 
while in the higher it forms the axis around which the membrane bones 
of the definitive jaw are arranged. 

''The hyoid arch lies between the spiracle and the first true gill cleft, 
in the region of the seventh nerve. It divides into an u'pper element, the 
hyomandibular cartilage, and a ventral portion, the hyoid proper* which 
may subdivide into several parts. In the lower elasmobranchs the 
hyomandibular and the rest of the hyoid arch are closely connected, but 

The Endoskeleton 


in the higher fishes the hyomandibular becomes more separated from 
the ventral portion and tends to intervene between the mandibular arch 
and the cranium, becoming a suspensor of the jaws. Still higher it loses 
its suspensorial functions, becomes greatly reduced, and apparently is 

Fig. 410 

A, Dorsal and B, Ventral views of cranium of Branchiosaurus salamandroides, 
C, Posterior view of cranium of Trematosaurus. Br, branchial arches; C, condyle; 
Ep, epiotic; F, frontal; /, Jugal; L.O., lateral occipital; M, maxillary; N, nasal; 
No, nostril; Pa, parietal; PI, palatine; Pm, premaxillary; P.o, postorbital; Pr.f, 
prefrontal; Pa, parasphenoid; Pt, pterygoid; Ptf, postf rental; Q, quadrate; Qj, 
quadrato-jugal; S.o, supraoccipital; Sg, squamosal; St, Supratemporal; V, vomer. 
(After Gadow.) 

D, Chondrocranium of a frog shortly after metamorphosis. fov, fenestra 
vestibuli; m, Meckel's cartilage; mtg, metapterygoid; nc, nasal capsule; ptgg, 
pterygoquadrate; tnas, tectum nasalis; tsyn, tectum synoticum; timed, taenia 
tecti medialis; III-V, nerve exits. (From Kingsley after Gaupp.) 

subsidiary to the sense of hearing or it may be lost, the question nof 
being decided. The hyoid proper becomes more or less intimately con- 
nected with the arches behind and also is largely concerned in affording 
a support for the tongue. 

"The branchial arches are all similar to each other in the lower ver- 
tebrates, but with the loss of branchial respiration in the higher groups, 
they tend to become reduced, the reduction beginning behind. Some may 
entirely disappear, others give rise to the laryngeal cartilages and the 
first may fuse with the hyoid. The first arch is in the region of the ninth 
nerve ; the others in that supplied by the tenth." 


Comparative Anatomy 

Here^ it will be necessary to review the frog skull very thoroughly, 
bearing in mind that, as in all vertebrates, the occipitals, the two sets of 

Fig. 411. 

Upper two views, skull of snake, an, angulare; av, articular; bo, basiocciptal ; 
bs, basisphenoid; d, dentary; eo, exoccipital; epo, epiotic; /, frontal; mx, 
maxillary; n, nasal; oo, opisthotic; p, parietal; pi, palatine; ptn, premaxillary; 
pro, prootic; ps, parasphenoid; pt, pterygoid; g, quadrate ; .ya, surangulare; so, 
supraoccipital; sq, squamosal; tr, transversum. (From Kingsley after W. K. 

Lower two views, skull of turtle. A, lateral view. B, from below, ang. 
angulare; art, articulare; au, orbit; b.sph., basisphenoid; ch, choanae; d, dentary; 
fr, frontal; ;', jugular; max, maxillae; occ.b, basi-occipital; occ.l, lateral occipital; 
occ.s., superior occipital; op.ot., opisthotic; pal, palatine; par, parietal;^-> 
postf rental;, premaxillae;, prefrontal; pt, pterygoid; g, quadrate; 
g.}., quadratojugal; sq, squamosal; vom, vomer. (After Schimkewitsch.) 

'^The frog does not have all the bones mentioned. All head bones that any verteibrate possesses 
are given in the list, so that the student must not expect to find any animal with all these structures, 
though he will find no vertebrate with head-bones not mentioned here. 

The Endoskeleton 


Fig. 412. 

Upper view, a diagram of a bird's skull, disarticulated. 
(After Gadow.) Membrane bones shaded. B.Oc, basioccipital; 
E.Oc, exoccipital; S.Oc, supraoccipital; Pa, parietal; Fr., frontal; 
Na., nasal; pm., premaxilla; M., maxilla; /m., jugal; Qj., quadrato- 
jugal; Qu., quadrate; PL, palatine; Pt., pterygoid; pe., periotic; Sq., 
squamosal; AS., alisphenoid; B.S., basisphenoid; O.S., orbitosphenoid; 
Pr.Sph., presphenoid; vo., vomer; iOs., interorbital septum; E., ethmoid; 
Se., nasal septum; De., dentary; Sp., splenial; An., angular; SA., 
supra-angular; Ar., articular; MK., Meckel's cartilage. 

Lower views, model of developing chick-skull. A, profile view, 
and B, hyoid apparatus, as seen from below, ang., Angulare;, 
basibranchial; b.hy., basihyal; c.aud., auditory (otic) capsule; cent., 
entoglossal cartilage; c.meck., Meckel's cartilage;, auditory 
columella; cor.hy., cornua of the hyoid; dent., dentary;, epi- 
branchial, 1; fr, frontal;^, ceratobranchial, 1; max, maxillary; nas, 
nasal; operc, opercular; pa, parietal; pal, palatine; pl.a.o., anteorbital 
plane; pl.s.sept., supraseptal plane; pl.sph.l., sphenolateral plane;, 
prefrontal;, retroarticular process; pr.tect.. Processus tectalis 
(roof-formiiig) ; p.sph., parasphenoid; pter, pterygoid; qu, quadrate; 
qu.}'., quadrate- jugal; s.ang., supraangular; sept.i.o., interorbital sep- 
turn; sq, squamosal; vom, vomer; zyg, zygomatic process. (After 


Comparative Anatomy 

sphenoids, and the ethmoids are the four groups constituting the car- 
tilaginous cranium. (Compare Figs. 407, 409, 410, 411, 412, 413, 414.) 

The occipital is divided into : 

A supraoccipital, an exoccipital, on each side, and a basioccipital. 
These four bones form the borders of the foramen magnum. 




foramen magnum 


— parietal 




lateral temporal 


postorbital bar 



lateral temporal 










Fig. 413. 

Dorsal view of the skulls of four representative vertebrates to show changes 
and reduction of the membrane bones of the roof. A, skull of an extinct amphibian, 
Capitosaurus, belonging to the Stegocephala; note the large number of membrane 
bones completely roofing the skull. B, skull of one of the most ancient reptiles, 
Seymourta, belonging to the Cotylosauria: the membrane bones are nearlj as 
numerous as in the amphibian, are similarly arranged, and completely roof the 
skull. C, skull of a modern reptile, the alligator; several of the membrane bones 
which were present in the extinct forms have been lost, and the roof has several 
openings. D, skull of a modern mammal, the dog, showing still greater loss of 
membrane bones. Membrane bones blank; cartilage bones stippled. (From 
Hyman's "A Laboratory Manual for Comparative Vertebrate^ Anatomy," A, after 
Reynolds, B, after Williston, by permission of the Chicago University Press.) 

The Endoskeleton 


The sphenoids are : 

The basisphenoid, extending forward to the sella turcica. 
The presphenoid, extending from the trabeculae to the ethmoid 

The alisphenoid, closely related to the basisphenoid. 

The orbitosphenoid, in close relation with the presphenoid. 

'-y mx. 

Fig. 414. 

A, A ventral view of the skull of a rabbit. al. External process of the 
alisphenoid; b.oc, basioccipital; b.sp., basisphenoid; c.a.m., external auditory 
meatus; ex.oc, exoccipital; f.m., foramen magnum; inc., incisors; ju, jugal; nir., 
molars; m.x., maxilla; oc, occipital condyle; pi., palatine; pm., premaxilla; pmr., 
premolars; pr.sp., presphenoid; pt., pterygoid; s.oc, supraoccipital ; ty.b., tympanic 
bulla; V, vomer;, zygomatic process of maxilla; sy.s., zygomatic process of 
squamosal. (From Borradaile.) 

B, A side view of a rabbit's slcull. Pmx., premaxilla; Na., nasal; Fr., frontal; 
Pa., parietal; Sq., squamosal; S.O., supraoccipital; Per., periotic; T., _ tympanic 
(the reference line points to the bony external auditory meatus, beneath it lies the 
inflated bulla); PO., paroccipital process. (From Thomson.) 

The ethmoids are divided as follows : 

Mesethmoid, lying medially. 

Ectethmoid, one on each side of the mesethmoid. 

The ectethmoid becomes the ectethmoid labyrinth which comes to 
lie between ectethmoid and mesethmoid. The turbinate bones are 
sometimes added here. 

From the otic capsule are developed various otic, or petrosal, bones. 

These are usually divided into : 

Prootic, lying in front of the ear. 

Opisthotic, lying behind, although usually meeting below with an 

272 Comparative Anatomy 

Epiotic and a sphenotic, in the teleosts and a few other forms 
developed from the lateral v^all of the otic capsule and lying in front and 

Pterotic, lying behind and directly above the horizontal semicircular 
canal of the ear. 

The otic bones usually fuse and form a petrosal bone in all the 
higher forms. This lies directly betw^een the lateral parts of the basi- 
occipital and the sphenoid. 

A ring of sclerotic bones is often formed from the sclera of the eye 
of birds and reptiles, though these never unite v^ith the regular bones. 
From the nasal capsules a lateral ethmoid often forms on the upper v^all, 
while the turbinate bones form on the medial and lateral walls. In man 
the single occipital bone is formed by the four occipitals mentioned 
above. The single sphenoid is a fusion of the six sphenoids mentioned 
above, the alisphenoids form the greater wings and the orbito sphenoids 
the lesser ones. (Fig. 408.) The ethmoid is similarly made up of the 
various ethmoids mentioned. 

The membranocranium gives rise to the following bones : 

Nasal, covering the olfactory region. 

Frontals, between the orbits. 

Parietals, on the same level with the otic capsules. 

Inter-parietal, unpaired, between parietal and supraocciptal. 

While these are practically all of the membranocranial bones in the 
roof of the cranium of the higher forms, others may appear in the lower 
groups. For example : 

Supratemporal, lying lateral to each parietal. 

Postfrontal, behind the orbit. 

Postorbital, forming the posterior wall of the orbit. 

Supraorbital, taking the place of the frontal in forming the superior 
or medial wall of the orbit. 

Prefrontal, bounding the orbit in front. 

Lacrimal, lying lateral to the prefrontal. 

Intertemporal, lying dorsal or medial to the alisphenoid. 

Postparietal, between parietals and interparietals. 

Epiotic, lying above each otic capsule and usually called the 

If, as in some fish, birds, and reptiles, the basilar plate and 
trabeculae fail to ossify, then the roof of the mouth, which is, of 
course, the frontal of the cranium, is also a membrane bone called the 
parasphenoid, while farther forward the vomers or plough-share bones 
are also membranous and lie in the nasal region. Both parasphenoid 
and vomers may bear teeth. 

As soon as bone begins to form, the pterygoquadrate changes 
considerably, becoming closely connected with the cranium in front. 
The middle portion disappears and the palatines, a pair of membrane 

The Endoskeleton 273 

bones, replace the disappearing part. The remaining portion of the 
visceral arch ossihes, usually though not always, forming two bones on 
each side, the paired anterior pterygoid and the paired posterior quadrate. 
These now become the suspensor of the lower jaw. In some forms such 
as the te-leosts and the reptiles, there appears an entire series of 
pterygoid bones. 

Outside of the pterygoquadrate, a second arch of membrane bone 
develops to form the functional upper-jaw in all bony vertebrates. This 
when fully developed consists of : 

Squamosal, underlying the quadrate. 

Quadratojugal, which follows immediately. 

Zygomatic, also called the malar or jugal. 


Premaxillary, which forms the tip of the jaw. 

Only the maxillary and premaxillary bear teeth. 

When, as in the higher forms, such as man, the roof of the skull is 
not continuous, but openings of various kinds are seen, such openings 
are known as fossae. The more common and constant are as follows : 

Infratemporal, being the most lateral. 

Supratemporal, which is separated from the foregoing by the 
squamoso-postorbital bars. 

Posttemporal, lying between the parietal, supratemporal, and 
occipital bones. 

Temporal, when infra and supra temporal fossae unite. 

Any of the bones mentioned above may fuse or disappear entirely 
in certain groups, while in others there may be connections quite differ- 
ent from the usual type. 

The lower jaw is by no means modified as extensively as the upper. 
Meckel's cartilage, by an ossification, gives rise to two bones in each 
half of the lower jaw. There is an articular bone (articulare) where the 
jaw meets with the quadrate, and at the tip where both sides unite — the 
symphysis^ — there may be a mento-Meckelian bone, although this does 
not occur often. The rest of Meckel's cartilage forms an axis about 
which the membrane bones of the lower jaw are arranged. These are 
as follows : 

(1) Dentary, surrounding the Meckelian in front and bearing teeth. 

(2) Splenial, on the inner side behind the dentary and often bear- 
ing teeth. 

(3) Angulare, on the low^er side usually extending back to the 
hinder end of the jaw. 

(4) Surangulare, lying on the outer posterior part of the jaw. 

(5) Coronoid, on the upper side of which muscles which close the 
jaws, are attached. 

(6) Goniale, also called antarticular or dermarticulare, lies on the 
medial and ventral sides of the articulare with which it usually fuses. 

274 Comparative Anatomy 

The dentary, splenial, and angulare are usually found, but very few 
vertebrates have them all. 

In the hyoid and branchial arches, the outside portions are known 
by the same names as the corresponding cartilages, membrane bones 
never being found here. 

The method by which the jaws are suspended varies. If the ptery- 
goquadrate is directly connected with the cranium, as in a few elasmo- 
branchs, the suspension is called amphistylic. If it is held in place by 
ligaments and the hyomandibular is interposed between the otic capsule 
and the hinder end of the jaw, it is called hyostylic; while, if the pterygo- 
quadrate is more or less fused with the cranium, as in all the higher forms 
beyond the fishes, it is known as autostylic. 


There are two types of appendages, namely : median, or azygos, 
which are found in aquatic vertebrates, and the regular paired appen- 
dages found in all other classes except the cyclostomes. Several theories 
have been advanced to account for the two types. One of these is that 
the two types of appendages have no relation to each other, and devel- 
oped independently, the pelvic and pectoral girdles being supposed to 
have originated from the gill arches, while the appendage bones have 
been derived from that portion which normally supports the gills. The 
other view assumes that two longitudinal folds ran the full length 
of the body behind the head (Fig. 415), each of these folds being sup- 

Diagrams showing A, the undifferentiated condition 
of the paired and unpaired fins in the embryo, and B, 
the manner in which the permanent fins are formed 
from the continuous folds. AF, anal fin; An, anus; BF, 
pelvic fin; BrF, pectoral fin; D, dorsal fin-fold; FF^ 
dorsal fin; RF, dorsal fin; SF, tail-fin; S, S, lateral folds 
which unite at S' to form ventral fold, (From Wieders- 

ported by a series of skeletal rods. The two dorsal and two ventral 
folds then fused to form the dorsal and ventral fin. The anal opening 
is, however, on the ventral side. Consequently, the caudal fins had to 
be formed from the ventral fusion behind the anal opening, while the 
portion anterior to the anal opening develops into the appendages proper. 

The Endoskeleton 


Both theories are unsatisfactory, the latter because there is no double 
origin of the dorsal hns, and the former, known as the gill-arch- theory, 
is unsatisfactory due to the fact that the paired appendages develop out- 
side the body musculature, while the visceral arches are always internal. 

All median appendages have the form of fins, and are termed dorsal, 
terminal (caudal), or ventral (anal). They may occur as a continuous 
fin, or they may be broken up with intervals between. Fins occur in all 
fishes, in the larval and tailed amphibians, and even in rather isolated 
groups, such as the Ichthyosaurs and the whales, although in the amphi- 
bians and the higher groups, there is no skeleton in the median fins. 

The skeleton of the fins usually consists of definite metameric car- 
tilaginous or osseous bars, each of which is divided into a proximal 
basale and a distal radiale. The basale often articulates or alternates 
with the spinous processes of the vertebrae. The radiale supports the 
fins proper. 

In the higher forms of vertebrates, when the skeleton of the fins is 
not composed of cartilage or bone, there is a horny substance known 
as elastoidin which forms a number of slender rods in greater number 
than the somites. They arise from the corium just below the epidermis, 
often being united in bundles, and thus form soft-finned rays, often 
replacing the radiale. 


An extinct shark had a pair of fins approximately in the region where 
pectoral and pelvic girdles normally form, but no satisfactory theory has 
yet developed as to how arms and legs were derived from this type of fin. 


Comparative Anatomy 

Fig. 416. 
Shoulder girdles. I. Ventral views of the shoulder girdles of various Anura. 
(Slightly enlarged.) 1, Bombinator igneus (a species of Frog), and 2, Bufo vul- 

The Endoskeleton 277 

The internal support of both shoulder and pelvic girdle consists of 
inverted arches across the ventral side of the body, the limbs of the 
arch extending dorsally. The part to which the limbs are attached is 
called the girdle. The arch itself always forms in cartilage, though 
membrane bone or dermal bone may be added. 

The typical girdle consists of three elementary parts, one dorsal and 
two ventral, all of which meet at the point of attachment of the limbs, 
and all contribute to form the socket, in the forelimb called the glenoid! 
and in the hindlimb known as the acetabulum. This shows why we 
consider pectoral and pelvic girdles and appendages homologues. 


In fishes, the shoulder girdle is more or less U-shaped, with the 
glenoid fossa at the dorsal end. Immediately dorsal to the fossa lies 
the scapular region. Quite often, the dorsal part of the scapular region 
is again divided, so that a supra-scapula is formed. In the skates, the 
supra-scapula articulates with the adjacent vertebrae; usually, however, 
the entire girdle lies free from the axial skeleton. A pair of clavicles form 
from the skin. These overlay the coracoid region of the girdle, and meet 
in the midline, while a cleithrum, a second bone above the glenoid fossa, 
forms. In some of the ganoids, it is the cleithrum which extends toward 
the midline so as to take the strain ; in fact, it is assumed that in higher 
groups, where the two halves of the cartilaginous girdle have separated, 
the separation is due to the stress laid upon these parts. 

In the higher ganoids and in the teleosts, the cleithrum increases in 

garis (Toad) as examples of the arciferous type. 3, Adult, and 4, metamorphosing 
Frog, Rana temporaria showing change from the arciferous into the firmisternal 
type. _ 5, Hemisus guttatum (narrow-mouthed toad). 6. Breviceps gihbosus (tailless 
amphibia). 7, Cacopus systoma (narrow-mouthed toad). Cartilaginous parts are 
dotted. Ossified parts are white. CI, clavicle; Co, coracoid; E, epicoracoidal 
cartilage; H, humerus; M, metasternum; O, omosternum; P, precoracoid; Sc, 
scapula; S.S., suprascapula. (From Gadow and Boulenger.) 

II. A, The skeleton of the pectoral fins and girdle of a dogfish, seen from the 
ventral side. (After Borradaile.) cor., Coracoid region; gL, glenoid surface; h.r., 
horny rays; mpt., metapterygium; mspt., mesopteryg[ium ; ppt., propterygium; 
rad, cartilaginous rays; sc, scapula. B. Ventral view of the shoulder-girdle 
and _ sternum of a Lizard, Loemanctus longipes. (After Parker.) 1. Inter- 
clavicle. 2. Clavicle. 3. vScapula. 4. Coracoid. 5. Precoracoidal process. 6. 
Glenoid cavity. 7. Sternum. 8. Sternal bands not united. 9. Sternal rib. 
C. Sternum and associated membrane bones of a Crocodile, C, palustris. (After 
Shipley and MacBride.) The last pair of abdominal ribs which are united with 
the epipubes by a plate of cartilage have been omitted. 1. Interclavicle. 2. 
Sternum. 3. Sternal rib. 4. Abdominal splint rib. 5. Sternal band. D. Lateral 
view of the pelvis and sacrum of a Duck, Anus boschas. (After Shipley and 
MacBride.) 1. Ilium. 2. Ischium. 3. Pubis. 4. Pectineal process, the rudi- 
ment _of^ the prepubis corresponding to the pubis of the Lizard. 5. Acetabulum. 
6. Ilio-ischiatic foramen. 7. Fused vertebrae. 8. Facet on which the projection 
on the _ femur, the trochanter, plays. E, The breastbone and shoulder girdle of 
a rabbit, seen from below and somewhat from in front. (After Borradaile.) 
acr., Acromion; c/., clavicle; cor., coracoid process; cp., capitulum; g.c, glenoid 
cavity; mb., manubrium; mcr., metacromion; sc, scapula; st.r., sternal portion 
of a rib; st. 2, st. 6, second and sixth sternebrae; tb., tuberculum; v.r., vertebral 
portion of a rib; x., xiphisternum; x.c, xiphoid cartilage. F. Sternum and sternal 
ribs of a Dog, Canis jamiliaris. (After Shipley and MacBride.) 1. Presternum. 
2.^ First sternebra of mesosternum. 3. Last sternebra of mesosternum. 4. 
Xiphisternum. The flattened cartilaginous plate terminating the xiphisternum is 
not shown. S. First sternal rib. 

III. Pectoral girdles of two types of ganoids. A, Acipenser (Sturgeon) and 
B, Polypterus (African ganoid), ct, cleithrum; cz', clavicula; dr, dermal rays; g, 
glenoid surface; r, cartilaginous radialia. (From Kingsley after Gegenbaur.) 


Comparative Anatomy 

size and usurps the function of the clavicle, while the clavicles themselves 

Other bones from the skin may develop, such as the supracleithra 
(post-temporals or supra-temporals). These connect the girdle vi^ith the 
skull; sometimes also post-clavicles and infra-clavicles develop. 

A review of the 
appendicular skeleton, 
as formed embryolog- 
ically in the chick and 
frog, will give one a 
good idea of the parts 
as they appear in am- 
phibia. In the reptiles, 
there is a considerable 
variation in the shoul- 
der girdle. In the 
turtle, its position in- 
side the carapace and 
internal to the ribs is 
supposed to be due to 
the fact that the girdle 
begins its development 
in front of the ribs, and 
later sinks to the posi- 
tion it is to occupy in 
adult life. The scap- 
ula, procoracoid, and 
coracoid are well de- 
veloped. The median 
ends of the latter are 
connected by a cartila- 
ginous epicoracoid. In 
other forms of reptiles, the procoracoid usually is reduced and the 
clavicle takes its place, though in the lizards, the procoracoid still 
remains in its reduced condition. Clavicles may or may not be present 
in turtles. If they are, they are represented by the epiplastron (Fig. 417), 
which is an element of the carapace. In the chameleons and crocodiles 
the clavicle is entirely lost. In limbless lizards, the girdles are greatly 
reduced, and in fact in the Ophidians, the girdle itself has completely 

In birds (Fig. 418), the scapula is formed as a sword-shaped bar 
overlying the ribs, while the coracoid extends from the glenoid fossa to 
the anterior end of the sternum. The procoracoid has entirely disap- 
peared. The two clavicles unite ventrally to form the wishbone, called 

Fig. 417. 

Tortoise skeleton Cistudo lutaria. Ventral side with plastron 
removed and placed at one side. C, costal plate; Co, coracoid; 
e, endoplastron; ep, epiplastron (clavicle); F, fibula; Fe, femur; 
H, humerus; Hyp, hypoplastron; Hpp, hypoplastron; //, ilium; 
Js, ischium; M, marginal plates; Nu, nuchal plates Py, pygal 
plates; R, radius; sc, scapula; T, tibia; U, ulna; Xp, xiphiplastron. 
(From Parker and Haswell after Zittel.) 

The Endoskeleton 


Fig. 418 — Skeletons of Rabbit and Bird. 
A. — The skeleton of a rabbit. 

acr., Acromion; cd.t., condyles for tibia; cm., calcaneus; cn.c, cnemial crest: 
fe., shaft of femur; fi, fibula; g. t., great trochanter; gr.t., premolar and molar 
teeth; h., head of humerus, fitting into glenoid cavity; hu, shaft of humerus; 
il., ilium; inc., upper incisor teeth of the left side; ind., lower incisor tooth; ij., 
ischium; ju., jugal bone; lac, lacrymal bone; mcr., metacromion; nix., maxilla; 
o.f., obturator foramen; oL, olecranon process; os., orbitosphenoid bone; pa., knee- 
cap; pis., pisiform bone; pu, pubis; ra., radius; sc, scapula; sp.s., spine of 
scapula; St., sternum; st.r., sternal ribs; sup., suprascapula; t.Z, third trochanter; 
ti., tibia; tro., trochlea; uL, ulna;, v.cer., v.L, v. sac,, caudal, cervical, 
lumbar, sacral, and thoracic regions of the backbone; v.r., vertebral ribs; x., xiphi- 
sternum; x.c, xiphoid cartilage; 11, foramen for optic nerve. The clavicle and 
hyoid are not shown. 

B. — The skeleton of a pigeon, seen from the left side. 

C.r., Fixed cervical rib; cr.^, free cervical ribs; cl., clavicle; cor., coracoid; d., 

280 Comparative Anatomy 

the furcula (Fig. 418, B.cL). This may either articulate with the 
sternum or He free. 

In the monotromes, the shoulder girdle is quite like that of the Hzard. 
This is also true of the young marsupials, but in the adult, it becomes 
quite like that in all other adult mammals. The coracoid in this instance 
is reduced to the small coracoid process definitely ankylosed to the ven- 
tral end of the scapula. The scapula is well developed with a crest 
called the spina scapulae on its external surface which in turn culminates 
in an acromion process (Fig. 416, II, E, acr.). The clavicle varies with 
the manner in which the limb is used. 

In the higher forms of mammals, the clavicle serves as a strong 
brace between shoulder and sternum. However, in the ungulates, in the 
whales, and in a few carnivores it has entirely disappeared. In some 
mammals it appears as a mere rudiment, without apparent functional 

Two small cartilaginous elements often intervene between clavicle 
and sternum, called episternalia or suprastemalia. Their homologies 
are unknown. 


The hip, or pelvic girdle (Fig. 419), is quite homologous to the shoul- 
der girdle, the acetabulum representing the glenoid fossa. The ilium 
represents the scapula, while pubis and ischium represent the procora- 
coid and coracoid. The gap or open space between pubis and ischium 
is known as the ischio-pubic fenestra. In the lower forms there is 
another opening, called the obturator foramen, through which the 
obturator nerve passes to the pelvis. In the higher forms, this usually 
unites with the ischio-pubic fenestra, the entire opening then being 
called the obturator foramen. 

In the lower forms, such as the fishes, the basalia are on the inside, 
and fused tO' form a single basal, through which the obturator nerve may 
pass. The radialia are on its distal surface. The basalia of the two sides 
do not meet, though there is often a small (or a pair of small) cartilage 
plates between them. These are supposed to be the homologues of the 
epipubis of the higher forms. There is no acetabular joint. 

In the ganoids and teleosts, ossification begins, but there are no 
epipubic elements. The pelvic fins may migrate so as to lie in front of 
the pectoral. 

dentary; Eu., Eustachian tube; e.oc, exoccipital; f.r., fenestra! recess; fe., femur; 
fi., fibula; fr., frontal; hu., humerus; i.o.s., interorbital septum; il., ilium; is., 
ischium; lac, lacrymal; mc. 1-3, metacarpals; mt. 1-4, metatarsals; n., nasal; o.f., 
obturator foramen; pa., patella; par., parietal; ph. 1-4, phalanges; pi., palatine; 
pm., premaxilla; p.c.p., postorbital process of frontal; pt., pter3'goid; pu., pubis; 
Pyg-, pygostyle; q., quadrate; r.c, radial carpal; ra., radius; s.o.h., suborbital bar; 
s.oc, supraoccipital; sa., supra-angular; sc, scapula; sq., squamosal; st., sternum; 
st.r., sternal ribs; ti., tibia; u.c, ulnar carpal; u.p., uncinate process; ul., ulna;, caudal vertebrae; v.r., vertebral rib; x.p., xiphoid process; sy., zygomatic process 
of the squamosal; /., //., foramina for first two cranial nerves; 1-3, first three 
cervical vertebrae. (From Borradaile.) 

The Endoskeleton 


The elasmobranchs have a true girdle, although there are no sepa- 
rate elements in it, and it does not ossify, there being but a continuous 
ischiopubic bar running from one acetabulum to the other with an 
elongated iliac process running dorsad. 

The pelvic girdle lies free of the vertebral column in all fishes, but 

isc.pu- ac. 

Fig. 419. 

A — The skeleton of the pelvic fins and 
girdle of a female dogfish. 
ac, Acetabular surface; bp., basiptery- 
gium; h.r., horny rays; il., iliac process; 
isc.pu., ischio-pubic region; rad., cartilagi- 
nous rays. 


B. — The pelvic girdle of a rabbit, 
from beneath. 
ac. Acetabulum; il., ilium; is., 
ischium; ob.f., obturator foramen; 
pu., pubis; syni., symphisis pubis. 
(From Borradaile.) 


C. — Anlage of pelvic girdle of 6-day 
chick embryo to show develop- 
//., ilium; Isch., ischiurt;; ph., 

pubis; pp., pectineal process. (After 


in animals that have to support the body-weight upon their limbs, the 
pelvic girdle becomes definitely attached to the sacrum by the develop- 
ment of one or more sacral ribs. 

In the mud puppy (Necturus, Fig. 375), the median cartilage extends 
forward as an epipubic process while from the antero-lateral portion of 
each pubic bone or cartilage a pectineal process extends. In the 

282 ^ Comparative Anatomy 

salamanders, to the extent of two or three somites, there is a cartilage 
formed independently of the pubis in the linea alba, called the ypsiloid 

In the frog and other anura, three pelvic bones are present, all 
of which participate in the forming of the acetabulum. The ilium, how- 
ever, is very long and the ischio-pubis strongly compressed so that the 
obturator foramen and ischio-pubic fenestra are absent. 

In reptiles, the pelvic bones are more solid and distinct than in any 
of the lower forms. The ilium is often expanded, the ischio-pubic fenes- 
tra large, and the ischium and pubis united from side to side by an 
epipubic cartilage or a modification of this, known as the ligamentum 
medium pelvis. 

In some turtles, the epipubic cartilage bounds the fenestra on the 
median side, but in all turtles, the fenestra and the obturator foramen 
are merged into one. In lizards, there may be a separate bone ossified 
from the posterior part of the epipubis. This bone is called the os cloacae 
or hypo-ischium. 

In legless lizards, the pelvis is greatly reduced, while all trace of it 
is lost in the snakes, except the boas and some opoterodonts (worm-like 
serpents). In the crocodiles, due to the oblique position of the pubes, 
the obturator foramen is very large. The pubes themselves do not unite 
with each other. There are cartilaginous tips on the medial end which 
may be separate epipubes. The lower end of the ilium also separates 
as a distinct bone. 

It is interesting to note that the pelvis of Dinosaurs has the ilium 
arranged quite similar to that in birds. The sacrum also is somewhat 
similar, due, apparently, to the upright position in which these animals 
walked. The ischia are elongated, extending backward, and often unite 
below, while the pubic bones extend forward and downward, and have 
strong post-pubic processes running parallel to the ischium, while often 
the ilium gives oiT an iliac spine near the acetabulum. 

The Pteryodactyls also had elongated ilia similar tO' the Dinosaurs. 
The ischium was then fused with the ilium so that the pubis took no 
part in the forming of the acetabulum. The pelvic opening was very 

In birds of the present time, the pelvic bones are fused, the ilium is 
quite long, and usually fused with the synsacrum, while the ischium and 
pubis extend backward. The pubes lie in the position of the postpubes 
of Dinosaurs and never meet below, except in ostriches. However, in 
the embryo, the pubes run forward only to gain their final position later 
on. There is a pectineal process which arises in the acetabular region 
and extends forward quite like the pubis in Dinosaurs. In the mammals, 
the obturator foramen and ischio-pubic fenestra are united ; all three 
pelvic bones unite to form the acetabulum, although the ilium and 
ischium may extend in such a manner as to exclude the pubis from 

The Endoskeleton 283 

taking part in the formation of the fossa. Often an acetabular or cotyloid 
bone is formed between the ilium and pubic bones, and this may fuse 
with any of the bones with which it comes in contact. 

The inter-pubic cartilage in marsupials and monotremes may or 
may not persist throughout adult life. When it disappears and the bones 
unite solidly, but do not definitely ankylose, such union is called a 

In these non-placental mammals (the marsupial and monotremes 
just mentioned) there are also marsupial bones which first form in car- 
tilage and then extend forward from each pubis in the ventral abdominal 
wall. Their homology is unknown. 


In those animals, such as fishes living entirely in the water, the 
appendages are called ichthyopterygia. These are always paired fins. 
When definite legs or arms are formed, as in all classes of tetrapoda, 
such limbs and their modifications are known as chiropterygia. It is 
commonly supposed that the limbs have developed from the fins, 
although no one has yet been able to explain the method by which it came 
about. All explanations, however, assume that certain parts of primitive 
fins were retained and others likely modified, or, that certain parts were 
lost, which were originally present, the remaining parts then becoming 
modified. The lower ganoids have a primitive form of fin, but with 
increasing complexity, there is a reduction of the basalia either by entire 
disappearance or by fusion. The remaining ones are then modified so 
that, in elasmobranchs of the present time, we find the basalia usually 
number three in the pectoral and two in the pelvic fins, being named 
from before, backwards, as the pro-, meso-, and meta-pterygium (Fig. 
416). The middle one is absent in the hind limbs. The radiales are 
jointed transversely so as to give more flexibility. If these are arranged 
entirely on one side of the basalia, they are called uniserial, but, if they 
occur also on the post-axial side, they are called biserial. The male 
elasmobranch has the pelvic fin divided into two lobes, the medial being 
called the clasper, or mixipterygium. 

The anterior portion of the pectoral fin may develop as a strong 
defensive spine, sometimes connected with the poison gland. In eels the 
pelvic fin is lacking. 


The legs (chiropterygia) of all tetrapoda are essentially alike (Fig. 
420). Each consists of several regions, comparable in detail with each 
other. The proximal is the upper arm (brachium), or thigh (femur), 
containing a single bone ; the humerus, or femur, in the fore or hind limb 
respectively. The next region, the forearm (antebrachium) , or shank, 


Comparative Anatomy 

Fig. 420. — Comparisons of fore-limbs and hind-limbs. 

1. Wing of a dove; c, carpals; h, humerus; mc, carpo- 
metacarpus; p.f-, primary feathers; r., radius; s.f., secondary 
feathers; u., ulna. 



•-t ^^ ^ ,..v . 

A, and B. — The fore-limb and hind-limb of a bird compared. 

H., Humerus; R., radius; U., ulna; r., radiale; u., ulnare; C, distal carpals 
united to carpo-metacarpus; CC, the whole carpal region; MCI., metacarpal of the 
thumb; 1., phalanx of the thumb; MCI I., second metacarpus; //., second digit; 
MC.III., third metacarpus; III., third digit. F., femur; T.T., tibio-tarsus; Fi., 
•fibula; Pt., proximal tarsals united to lower end of tibia; dt., distal tarsals united 
to upper end of metatarsus, forming a tarso-metotarsus {T.MT.); T., entire tarsal 
region; MT.I., first metatarsal, _ free; I. -IV., toes. 

C, D, E, F, G. — Anterior limb of man, dog, hog, sheep, and horse; Sc, Shoulder- 
blade; c, coracoid; a, b, bones of forearm; 5, bones of the wrist; 6, bones of the 
hand; 7, bones of the fingers. 

H, I, J, K, L. — Posterior limb of man, monkey, dog, sheep, and horse: 1, Hip- 
joint; 2, thigh bone; 3, knee-joint;" 4, bones of leg; 5, ankle-joint; 6, bones of foot; 
7, bones of toes. (A, B, after Thomson, C to L, after Le Conte.) 

(crus) contains two bones, a radius, or tibia, on the pre-axial, and an 
ulna, or fibula, on the postaxial side. Next follows the podium, or hand 
(manus), in front and the foot (pes) behind, each consisting of three 
portions. The basal podial region, the wrist (carpus), or ankle (tarsus), 
consists of several small bones ; the second division (metapodium) is the 
palm (metacarpus), or instep (metatarsus), and lastly come the fingers, 
or toes (digits), each digit consisting of several bones, the phalanges. 
These separate parts are included in the accompanying table, in which 
the terms given to the separate elements of the wrist and ankle of man 
are included. 

The Endoskeleton 


Upper arm (Brachium) 


Fore arm (Antebrachium) ^ 










Palm f 

(Metapo- J 

dium) (^ 

Fingers (Phalanges) 


Humerus = Femur Thigh 

Radius =^ Tibia 

Ulna = Fibula 
Radiale = Tibiale 

Intermedium = Intermedium 

Ulnare = Tibiale 
Centralei + 2 = Centralei + 2 

Carpale^ — Tarsale^ 

Carpale^ = Tarsale- 
Carpale"^ = Tarsale*^ 

C Carpale* = Tarsale'* 

[ Carpale^ = Tarsale^ 

Metacarpale^"" = Metatarsale^ 




















„ Instep 

(Phalanges) Toes 

The basal podial region, which is nearly typical in some reptiles, 
urodeles (Fig. 421) and man, consists of three rows of bones: a proximal 
of three bones ; a radiale, or tibiale, on the anterior side ; an ulnare, or 
fibulare, on the other ; and an intermedium (not shown in the figure) 
between them. The distal row now consists of five carpales, or tarsales, 
numbered from the anterior side. 

The third row is composed of one or two centrales between the other 
rows. The metapodials (Metacarpals and Metatarsals) and the digits, 
also numbered from one to five, have in some cases special names. The 


Comparative Anatomy 

thumb (digit 1) is the pollex, the corresponding great toe is the hallux, 
while the fifth digit is called minimus, the second finger in the hand the 
index, and the fourth the annulus. 

From this typical condition all forms of chiropterygia — legs, arms, 
wings — are derived by modification, fusion, or disappearance of parts. 

Fig. 421. 

-The skeleton of the hind-limb of a frog-tadpole compared with the 
left fore- and hind-feet of a rabbit. 

A, fore-foot of rabbit; B, hind-foot of rabbit; C, hind-limb of frog- 
tadpole shortly before metamorphosis. 
In A and B: 

a, Astragalus; c.\, first distal carpal or trapezium; c.2, second 
distal carpal or trapezoid; c.3, third distal carpal or os magnum; c.4,5, 
fused fourth and fifth distal carpals or unciform; ce., centrale; ce^., 
centrale oi hind-foot or navicular; cm., fibulare or calcaneus; im., 
intermedium or semilunar, mc, metacarpals; met., metatarsals; ph., 
phalanges; ra., lower end of radius with its epiphysis; r.c, radiale or 
scaphoid; t.2, second distal tarsal or mesocuneiform; t.Z, third distal 
tarsal or ectocuneiform; t.A, 5, fused fourth and fifth distal tarsals or 
cuboid; u.c, ulnare or cuneiform; uL, lower end of ulnar with its 
epiphysis; I.-V., digits. (From Borradaile.) 
In C: 

centr., centralis; cl.calc, _ cartilaginous calcaneous ligament; 
c.l.tars.s., supplementary cartilaginous tarsal ligament;, sesamoid 
cartilage; F, fibula; fib, fibulare; pr.h., prehallux; ses, sesamoid bone; 
T, tibia; t.ach., tendon of Achilles; tars, II and ///, second and third 
tarsals; tib, tibiale; I-V, phalanges. (After Tschernoff.) 

The more distal a part is, the more variable it is ; reduction takes place 
on the margins of the appendage, the axial portions being the last to 
disappear. Occasionally, in various groups (amphibia, mammals) there 
occur what are interpreted as rudimentary additional digits — prehallux, 
prepollex, and postminimus — but their status is uncertain. There are 
also certain membrane-bones developed in the appendages, such as the 
patella (knee-cap) in some reptiles, birds, and many mammals, in the 
tendon that passes over the knee joint ; the fabellae in the angle of the 
knee of a few mammals, and the pisiforme in the carpus of man and 
some other mammals. 

The Endoskeleton 287 

We have already seen that in the frog the radius and ulna, as well 
as the tibia and fibula, are fused together while the tarsals are consider- 
ably elongated. Such fusion is not uncommon in many animals. The 
extent of fusion varies, however, considerably. In the reptile limb there 
is an intratarsal joint so that the motion of the foot upon the leg lies 
between the two rows of tarsal bones, instead of between the tarsals and 
the bones of the shank. This is quite similar to the condition in birds. 

Although limbs are lacking in the snakes and in some of the lizards, 
nevertheless, there is a considerable increase in the number of phalanges 
in those reptiles where limbs do occur, while the more proximal bones 
shorten. In some of the ichthyosaurs there may be as many as a hun- 
dred phalanges in a single digit. 

The skeleton of pterydactyls shows the fifth digit greatly developed, 
which forms a definite support for the wings, while the other digits 
remain more or less normal. In birds the wings are considerably modi- 
fied (Fig. 420) although the structure is practically normal up to the 
region of the carpus. The carpal bones are greatly reduced by fusion, 
while the metacarpals and digits, no matter what their modification, are 
only three in number. 

Embryological studies of the chick show us that, although the first 
digit begins to develop, it is entirely lost, and the fifth metacarpal, which 
is present in the embryo, fuses early with the fourth, so that the digital 
formula is II, III, IV. Then, too, there is an extensive fusion of the 
bones of the tarsus and foot; the ankle joint is intratarsal, the basal 
row of tarsal bones fuses with the tibia, while the fibula is considerably 
reduced to form the tibio-tarsus. The tarsales unite in the same way 
with the fused metatarsals to form the tarso-metatarsus. 

There are hardly ever more than four toes, but the number of 
phalanges increases from two in digit II, to five in digit V. Ostriches 
only have two toes, and many other birds three. In the mammals, 
especially in the higher forms, there is considerable motion of both hand 
and foot ; rotation in the hand is especially noticeable by the motion 
of the lower end of the radius around the ulna. In the whales the basal 
part of the forelimb is greatly shortened, while there is considerable 
multiplication of the phalanges. The hind limb is entirely lacking in 
some whales, while in others there are two vestigial bones, supposed to 
be the femur and tibia, imbedded in the muscles of the trunk. 

A supra- or entepicondylar foramen frequently perforates the inner 
lower end of the mammalian humerus while in many forms the ulna is 
fused with the radius in varying degrees. However, the ulna, whether 
fused or not. always has on its proximal end a strong olecranon process 
which extends beyond the elbow joint for the attachment of the extensor 
muscles of the forearm. 

The earliest prominences for the attachments of the muscles on the 
femur are known as trochanters. They vary from one to three. The 


Comparative Anatomy 

Fig. 422. 

Cyclostomes, as exemplified by the marine lamprey, (Petromyson marinus), 
from 60 cm. to 1 m. long, of European, West African, and North American waters, 
which goes up stream of the river in spring to lay eggs in the calm waters, and 

The Endoskeleton 289 

patella, or knee-cap bone, is analogous to the olecranon process, though 
it never joins the other bones. 

The ankle joint in mammals is never intratarsal, but always between 
tarsal and crural bones. 

Where the bones of the foot rest on the ground as in man and in 
the bear, such a foot is known as plantigrade. Where the toe of the 
foot includes only the distal phalanges such as in the dog and the cat, 

comes down again toward the sea in autumn; and the Planarian Lamprey, 
{Petromyzon planari), from 20 to 30 cm. long which inhabits the calm waters 
entirely, and is commonly found in rivers. 

Fig. 1. A lamprey {Petromyson planari) with its mouth fixed to a rock. Pg., 
Genital papillae. 

Fig. 2. Anterior part of the body of Petromyson marinus, showing the seven 
branchial openings and the buccal cupping glass surrounded by little papillae. The 
olfactory opening lies in front of the eye. 

Fig. 3. Section through the anterior region of Petromyzon marinus. The sec- 
tion, slightly oblique, is nearly sagittal toward the front; it deviates dorsad and 
downward in order to take in the last of the left branchial sacs; r;&., buccal cupping 
glass; ca., ringed cartilage carrying the principal teeth; cf., cartilaginous pieces of 
the face; /, lingual sucker, (the posterior part of the sucker has not been taken into 
the section), is shown surrounded by its sheath; ml., muscles of the lingual sucker; 
ph., pharynx; oe., oesophagus; m.oe., sphincter closing the entrance of the oesopha- 
gus;, branchial aqueduct, showing the seven openings to the branchial sacs; 
va, valvular apparatus closing the entrance to the aqueduct; br.^-br.,, branchial 
pockets continuing into the coelomic peribranchial cavities, the one being separated 
from the other by septa; C, heart; the auricle has been raised partly to show the 
openings by which it communicates with (1) the ventricle, (2) with the sinus 
venosus, sv., as one sees them both from behind; vc, entrance of the cardinal veins 
into the sinus; ;., jugular vein; vh., hepatic vein; tao., aortic trunk, with the 
conus arteriosus and its valvular apparatus at the base of the trunk; ao., aortic 
roots, reuniting on a level with the fifth branchial opening to form the aorta; n., 
nostril; sh., bottom of the hypophysial sac with a valvular sac lying before it; cer., 
brain; rn., medulla; cd., dorsal cord; /., liver; ov., ovary; p., posterior cul-de-sac of 
the branchial enclosure which protects the pericardium. 

Fig. 4. Mouth of the Marine Lamprey, de., teeth of the head of the lingual 
sucker; di., lower median tooth; ap., principal lateral teeth grouped in twos; da., 
accessory teeth; ph., sensory papillae of the buccal lip; os., cutaneous sensory 

Fig. 5. Anterior region of the skeleton; ca., ringed cartilage carrying the 
principal teeth; d., cf., cartilaginous parts of the face; cr., brain box; /., lingual 
cartilage; oL, olfactory capsule; cd., dorsal cord; an., the two neural arches, anterior 
and posterior, of the same metamere; cbr., branchial enclosure; a, cartilaginous 
rings surrounding the opening of the external gills; p, posterior cul-de-sac of the 
branchial enclosure; holding the heart (after Parker); the left half only of the 
skeleton is represented. 

Fig. 6. Section of a horny tooth (odontoid) of lamprey; ep., buccal epithelium; 
Pa., dermal papillae; D, tooth in use; d, replacing tooth, in process of develop- 
ment;^, horn producing cellules. (After Warren.) 

Fig. 7. Sagittal section of the Pineal Eye: ep., epidermis; de., dermis; op., 
pineal eye; np., pineal nerve; pp., parapineal eye; ha., commissure and habenular 
ganglion; ch, chorioid curtain and lamella concealing the mesencephalon; ca., anterior 
commissure; on the walls of the thalamencephalon; cp., posterior commissure; cr., 
cranial cartilage. (After Studnicka.) 

Fig. 8, Section of a branchial pocket, passing through internal and external 
openings. (By reason of the situation of these two openings, the section of the 
left side of the figure is practically on a plane which places the face to the observer 
and forms an angle of 45° with the median plane of the animal; the rest of the 
section, which is only drawn in, is entirely transverse, and therefore seems short- 
ened), cd, dorsal cord; an, neural arch; m, section of the medulla; g, fatty tissue 
completing the padding of the neural canal; oe, oesophagus and beneath, the aorta; 
abr, branchial aqueduct;, aortic trunk; /, lingual cartilage and its muscular 
sheath; ;, jugular vein; cae, appendages of the general cavity; cbr., section of the 
divers pieces of the branchial enclosure; o, internal branchial orifice at the interior 
of the pocket; o\ external branchial opening, with its threefold valves and cartilag- 
inous ring;, wall of the branchial pocket; fbd., branchial leaves; par., peri- 
branchial cavity slightly taken in section; mu, muscles. 

(1) Vignettes of the title: Scheme of the respiratory apparatus of two cyclos- 
tomes seen from the ventral surface: the oesophagus and the respiratory sacs 
of the left side (G) only are represented; the horizontal flesh is turned toward the 
caudal end of the animal; the oesophageo-cutaneous canal, which exists only on the 
left side is figured in discontinued tracts. To the left the respiratory apparatus of 
Myxine is seen (six branchial pockets with efferent canals running to a single 
opening) to the right, the respiratory apparatus of Bdellostoma polytrema (10-14 
branchial pockets). (After Dean in Goodrich.) (From the chart of Remy Perrier 
& Cepede.) 

290 Comparative Anatomy 

it is called a digitigrade foot, while, if the animal, such as the horse or 
cow, walks upon hoofs which are homologous to the nails on the hands 
and feet of higher foims, such a foot is called an unguligrade foot. 

There may be variations and fusions in all these animals. For 
example, in the horse, it is only the third digit which persists in a func- 
tional condition. 



The cranium lies entirely beneath the brain and forms neither side 
nor roof for the latter (Fig. 422). The cranial cartilages are sometimes 
said to be homologous with those of the embryonic fish skull. 


The investing bones are closely applied to the roof and floor of the 
chondrocranium and modify its form considerably by projecting beyond 
the cartilaginous part so that apertures and cavities are concealed (Fig. 
40/). The large frontals, which cover the greater part of the roof of the 
skull, conceal the fontanelles and furnish roofs to the orbits. Immediately 
behind the frontals is a pair of very small parietals; in front of them 
is an unpaired supra-ethmoid to the sides of which are attached a pair 
of small nasals. On the ventral surface is the large parasphenoid which 
forms a kind of clamp to the whole cartilaginous skull-floor ; and in front 
of, and below, the parasphenoid is the toothed vomer. Encircling the 
orbit is a ring of scale-like bones, the sub-orbitals. 


The fish skull (Fig. 409) is covered above and below by numerous 
dermal investment bones which are much like those of the primitive 
extinct amphibia Stegocephali. By boiling, all the investment bones 
are loosened and, when removed, a chondrocranium like that of the dog- 
fish is seen. 

In fishes there are primary and secondary structures in the jaw as 
in the cranium. The primary upper jaw (palatoquadrate) is considered 
homologous with the upper jaw of the dogfish. It does not, however, 
remain cartilaginous but is ossified by five replacing bones : the toothed 
palatine in front which articulates with the olfactory capsule; the ptery- 
goid on the' ventral edge; the mesopterygoids on the dorsal edge of the 
original cartilaginous bar, and the quadrate at the posterior end of the 
latter. These bones do not enter into the gap and, consequently, do not 
constitute the actual upper jaw of the adult fish. External to them are 
two large investing bones, the premaxilla and the mnxilla, which 
together, form the actual or secondary upper jaw. They both bear teeth. 

The Endoskeleton 291 

A small scale-like bone, the jugal, is attached to the posterior end of 
the maxilla. 

Ihe lower jaw is quite similarly modified. The articulare articu- 
lates with the quadrate and is contmued forward by a narrow pointed 
rod of cartilage which is really the unossified distal end of the primary 
Meckel's cartilage (the primary lower jaw). The articulare is the ossi- 
fied proximal end ; therefore, a replacing bone. Then there is a large 
toothed investing bone which ensheaths Meckel's cartilage and forms 
the main part of the secondary lower jaw. This is the dentary. There 
is aiso a small investing bone, the angular, which is attached to the 
lower and hinder end of the articulare. 

The upper jaw connects with the cranium partly by the articulation 
of the palatine with the olfactory region and partly by means of a sus- 
pensorium. formed of two bones separated by a cartilaginous interval. 
The larger, usually called the hyomandibular, articulates with the audi- 
tory capsule by a facet, and the small pointed symplectic fits into a 
groove in the quadrate. Both bones are attached by fibrous tissue to the 
quadrate and metapterygoid, so that in this way the suspensorium and 
palatoquadrate together form an inverted arch which articulates freely 
in front with the olfactory and behind with the auditory capsule. This 
gives rise to an extremely mobile upper jaw. 

The chondrocranium is a solid one-piece capsule which completely 
encloses the brain and the principal sense organs. The cranium proper 
is fused with paired nasal capsules and paired auditory capsules. 

Closely associated with the skull, but not fused with it, is the man- 
dibular skeleton, consisting of an upper jaw (pterygoquadrate cartilages) 
and a lower jaw (Meckel's Cartilage). Back of the jaw are the visceral 
arches. These are composed of upper and lower parts like the jaws. 
The first pair is specialized as the hyoid arch, the five others are the 
more generalized branchial arches that afiford support for the gills. 


The skull (Fig. 410) articulates with the atlas by two condyles 
which are formed by the exoccipitals. There is an auditory columellar 
apparatus fitting into the fenestra ovalis. 


The skull (Fig. 412) is rounded, has large orbits, and the facial 
bones are extended out upon the beak. The quadrate is movable and 
articulates with the squamosal. There is a single occipital condyle. 
There are no teeth in modern forms. The cervical vertebrae have pad- 
dle-shaped articular surfaces which give the neck great flexibiHty and 
thus make the beak a very versatile instrument. 

292 Comparative Anatomy 


The special features in the turtle skull (Figs. 411, 413) are these: 
Teeth are absent; the maxillary, premaxillary, and dentary bones are 
covered w^ith hard, chitinous sheaths which form the upper and lower 
members of the cutting beak; the vomer is a single unpaired median 
bone, and there are no lachrymals or ectopterygoids. The pterygoids 
send wings of bone inward. The wings and the palatines form a con- 
tinuous roof of the mouth ; the supraoccipital is prolonged backward 
into a large, narrow process upon which are inserted the heavy neck 
muscles. All of these bones, even the quadrate, are firmly united into 
a solid cranium. There is only one occipital condyle. 


The skull of the mammal (Figs. 413, 414) is more compact than that 
of lower forms ; consequently, it contains fewer elements than the skull 
of reptiles. The following reptilian bones are not found in the adult 
mammalian cranium : pre- and post-orbitals, pre- and post-frontals, 
basi-pterygoids, quadrato-jugals, and supra temporals. The lower jaw 
is reduced to a single pair of bones in the mammal, the angulare, splenial, 
and articulare being absent. The latter bone is often said to have been 
drawn in to form the malleus of the ear bonelets, the quadrate has been 
drawn in to form the incus bonelet, while a remnant of the hyomandibu- 
lar cartilage forms the stapes. The whalebone whale (baleen whale. 
Fig. 392), shows the highest type of the so-called adaptive specialization 
among mammals. Here the teeth are rudimentary and functionless 
though present in the young. In the adult, they are replaced by baleen. 
The nostrils are paired, the skull symmetrical, the sternum is single, 
while the ribs are one-headed and articulate only with the transverse 
processes of the vertebrae. 


The vertebrae develop at the intersection of the myosepta with the 
mesenchyme that surrounds the notochord and neural tube. Each indi- 
vidual vertebra is formed by the union of the two caudal halves of the 
two sclerotomes of one segment with the cephalic halves of the two 
sclerotomes of the next succeeding segment (Fig. 305). The vertebrae 
therefore alternate with the myotomes. 

As the vertebrae and ribs are first formed in cartilage produced by 
the activity of mesenchyme, so also, bones which form later are true 
cartilage bones. In the elasmobranchs, the entire skeleton is made up 
of cartilage with only a slight impregnation of calcareous matter. 

Each vertebra begins as four pair of cartilages (called arcualia) 

The Endoskfxeton 


which surround the notochord. Dorsally these are an anterior pair of 
basidorsals and a posterior pair of interdorsals. Ventrally there is an 
anterior pair of basiventrals and a posterior pair of interventrals. 

In some fish and in extinct Amphibia and reptiles, these cartilages 
remain more or less separate. In most vertebrates, however, parts are 
lost, while the remaining portions fuse together to form a single vertebra 
which is then composed of a centrum (which encloses the notochord) ; 
a dorsally directed neural arch (which encloses the spinal cord) ; and a 
haemal arch (enclosing the blood vessels). (Figs. 352, 404.) 

■ The neural arch is made up of the fused basidorsals and the haemal 
arch of the fused basiventrals, while the centrum develops from varying 
parts in diilerent groups of animals. 

In the elasmobranchs, the centrum is formed within the notochordal 
sheath. Thus is formed a chorda! centrum as contradistinguished from 
that of nearly all other vertebrates where the centra are produced by the 
fusion of certain arcualia to form a perichordal or arch centrum. 

There are two kinds of ribs : those which arise at the intersection 
of the myosepta with the horizontal skeletogenous septum (true or 
intermuscular ribs), (Fig. 423, q), and those which arise at the inter- 

Fig. 423.— Diagram to show the skeleton-forming septa in the trunk region of a 


a, skin; b, neural tube; c, notochord; d, blood vessel; e, dorsal skeletogenous 
septum; /, ventral skeletogenous septum; g, horizontal skeletogenous septum; h, 
myoseptum; i, epaxial part of the myotome; /, hypaxial part of the myotome; k, 
coelom; I, intestine; m-p, cartilages from which the vertebrae are formed; tn, basi- 
dorsal; n, interventral; o, basiventral; p, interdorsal; q, intermuscular rib; r, sub- 
peritoneal rib. Note the positions of the vertebral cartilages and ribs with respect 
to the skeletogenous septa (From Hyman after Goodrich.) 

section of the myosepta with the ventral skeletogenous septum or its 
derivatives (false or subperitoneal ribs), (Fig. 423, r). Teleosts develop 
the latter type, while all other vertebrates develop true ribs. 

Some fishes (such as trout, salmon, herring and polypterus), how- 

294 Comparative Anatomy 

ever, develop both types of ribs and even additional ones at various 
levels of the myosepta. 

The veitebrae are connected with each other by a strand of noto- 
chordal tissue that perforates all the vertebrae like the string through 
a chain of beads. 

The fins have ray-like supports of cartilage, and the pectoral and 
pelvic limb-skeletons are supported upon simple horseshoe-shaped pec- 
toral and pelvic girdles, each composed of a single piece of cartilage. 


A short cervical and sacral region appear in Amphibia, the cervical 
becoming longer in the higher forms of vertebrates. The pelvic girdle 
lies free in fishes, but in all other vertebrates it is immovably attached to 
the sacrum. Three fingers develop early in the amphibian foot although 
a fourth appears quite late in development (Fig. 421). This fourth finger 
lies w^ell dov/n on the ulnar side of the hand. Then a rudiment of the 
fifth (the little finger) appears as a mere bump. The thumb, index, and 
second finger, therefore, seem to be phylogenetically the oldest digits. 
This is important in connection with the loss of fingers in other verte- 
brates, as the last to develop is usually the first to be lost. In the amphi- 
bian we find feet instead of fins. This brings a change in the type 
of movements in the animal, for with fins, an animal can only paddle 
backwards and forwards. The muscles are, therefore, decidedly differ- 
ent, and nearly all trace of the segmental arrangement in them is lost. 
Animals which live on land are relatively heavier than those which live 
in water, so there is need of a much more rigid axial skeleton as well as 
stronger limb girdles, and limb skeleton. This condition is brought 
about by a more complete ossification of the parts of the skeleton that 
bear the most weig^ht. Exoskeleton parts also tend to disappear so that 
in modern amphibia the exoskeleton is entirely absent with the excep- 
tion of the Caecilians where it is rudimentary. In the Stegocephaliins 
there is a head armor while the exoskeleton is lacking on the rest of the 
body. The sternum first appears in amphibia. 


In reptiles, birds and mammals the cervical region is longer than 
that of the Amphibia and the trunk region is divided into an anterior 
thoracic region with long ribs and a more caudal lumbar region with 
short ribs or with none. 

In all vertebrates, rudimentary ribs are usually found on the cervical 
and sacral vertebrae when these regions are present. 

Fossil remains show that there were many more plates and scutes 
on the turtles of the past than on those of the present. Both longitudinal 
and transverse rows of elements have disappeared so that the whole 

The Endoskeleton 


system is now greatly simplified. Most species of turtles today show a 
certain percentage of individuals with supernumerary scutes and plates. 
(Fig. 424.) 


Fig. 424. Various plastra and their horny shields. 

1, Testudo ibera; 2, Macroclemmys temmincki; 3, Cinosternum odoratum; 
4, Sternothaerus nigricans; 5, Chelodina longicollis ; 6, Chelone mydas. a or 
an, anal shield; abd, abdominal shield; / or fern, femoral; g or gul, gular, 
unpaired in 3; h or hum, humeral shield; i or int.g, intergular; im, infra- 
marginals; nt, marginals; p or pect, pectoral; x (in 1), inguinal shield consti- 
tuting with the axillary xx, the last trace of inframarginals. (After Gadow.) 

In the trunk region the vertebrae are rigidly united to the narrow, 
padd^e-like ribs (Fig. 425). There are eight cervical, ten thoracic, two 
sacral, and a variable number of caudal vertebrae, which are procoelous 
in form (Fig. 404). A peculiarity of the turtle is that both pectoral 
and pelvic girdles are inside, instead of outside, the ribs. Tliey actually 
arise from primordia internal to the ribs so it is not a case of migration. 
No one has yet been able to give a satisfactory explanation of this fact. 

The pectoral girdle (Figs. 416, 417) is made up of a triradiate group 
of flattened bones : the scapula, the procoracoid, and the coracoid, the 
last being the largest. These three bones unite to form a socket which 
receives the head of the humerus. The pelvic arch is more compact. It 
consists of pubis, ischium, and ilium, which unite to form the acetabulum 
for the head of the femur. Membrane bones are never found in the pelvic 
girdle of any animal. 


The sternum is keeled (Figs. 416. 4^8), except in such birds as the 
ostrich, and the ribs have uncinate processes (Fig. 418, B, u.p.) except in 
Screamers (members of the family Palamedeidae). The trunk vertebrae 


Comparative Anatomy 

Fig. 425. 

A. Diagrammatic transverse section through the shell of Testudo. The horny 
shields have been removed from the right side. On the left side one can see the 
neural, costal, marginal, and pectoral shields. The bony dermal plates are dotted. 
Cap, capitular portion of rib; Sp.C, position of spinal cord. 

B. Vertical section through part of the shell, magnified and diagrammatic. 
B, Bony layer of cutis; L, leathery layer of the cutis; M, cells of the Malpighian 
layer; P, star-shaped pigment-cells; Sc, stratum corneum composing the horny 

C. Diagram of skeleton of Testudo elephantopus, after removal of the left half 
of the carapace. The plastron is indicated by a section through the middle line. 
F^, femur, foreshortened; Fi, fibula; H, humerus; //, ilium; Is, ischium; P.P., 
pubis; R, radius; Scap, scapula; Tb, tibia; u, ulna; 3, third cervical vertebra; 1, 
3, 5, first, third and fifth fingers; XIII, thirteenth (fifth thoracic) vertebra. (After 

are mostly fused. There are three or four pre-caudal vertebrae with 
terminal pygostyle (Fig. 418), two cervical, and three to nine thoracic 
ribs, the latter attached to the sternum. The pectoral girdle is made up 
of paired, blade-like scapulae, paired coracoids which unite with the 
sternum, and three clavicles fused in the middle to form the "wishbone" 
or furcula. The pelvic girdle is a solid bone, composed of the fused 
ischia, ilia, and pubes. The pelvis is firmly fused with the sacral verte- 
brae. The leg skeleton consists of a large femur, a slender fibula, and a 
long, stout tibiotarsus, made up of the fused tibia and proximal tarsal 
bones ; the ankle joint is between the tibio-tarsus and the tarso-meta- 

The foot has four digits of which the hallux usually is directed 

The Endoskeleton 297 


The coracoid portion of the pectoral girdle (Fig. 416) is reduced 
to a small coracoid process in all placentals while the scapula of all 
mammals possesses a spinous process. 

There are usually paired clavicles and a median unpaired interclav- 
icle in all land mammals. These are membrane bones. 



IT will be remembered that all multicellular animals pass through a 
blastula stage, consisting of a hollow sphere composed of a single 
layer of cells, which then indents to form a gastrula. 

This means that there are now two layers of cells where there was 
only one before. The outer layer is called the ectoderm and the inner 
the endoderm. The indented end closes up, leaving a hollow tube com- 
posed entirely of endoderm in the center, which, due to its being used 
for other purposes than the ectoderm, and lying within the body, under- 
goes totally different experiences than does the outer part of the body, 
and these different experiences modify its structure. This hollow tube 
is the primitive digestive tract. It will thus be seen that the digestive 
apparatus is the very first one of the various systems of an organism 
to differentiate. 

This distinctive cavity is called the gastrocoele. In the lower inver- 
tebrates, this gastrocoele remains as a blind cavity with but a single 
opening. It is among the worms that it first becomes converted into a 
complete canal by the formation of an anal opening. In animals up to 
this stage, the same opening serves both for ingestion and egestion. 

What is considered a distinct advance in the development of multi- 
cellular animals is the development of a coelom, or body cavity, lying 
between the digestive tract just mentioned, and the body wall. Up to 
the time this coelom has developed, the body of the animal consists of a 
single tube and its wall. But after the coelom has developed, there is 
established a secondary open space between the hollow digestive tube 
and the body wall. 

The coelom is developed by protrusions, or diverticula, pushing off 
from the original digestive tube (Fig. 426). This means that the diges- 
tive canal of the higher animals only represents a portion of the digestive 
system of lower animals. 

Another departure from the lower organisms consists in the fact 
that the mouth and anal opening are not developed in the same way in 
the vertebrates as they are in the lower forms of animals. 

In the lower forms, after gastrulation, the indented end remains 
open, thus serving as both mouth and anal opening at the same time. 
In the higher forms, however, this indented end closes so that there is 
a completely closed hollow tube composed of endoderm on the inside of 
the body. To form the rnouth and anal opening, a new indentation at 
both the cephalic and caudal ends takes place. 

Digestive System 


This indentation, coming from the outer layer of the body, means 
that mouth and anus are composed of ectoderm and not endoderm as is 
the central digestive tube. After the indentation has gone far enough, 
the thin plate of cells separating the central digestive tube from the 
mouth and anus breaks through, so that a continuous opening is formed 
from the mouth through the digestive canal to the anal opening. 

All the additional structures that go to make up the digestive system 
as well as the respiratory system are formed by inpushings or outpush- 


Fig. 426. 

I. Diagrams to show method of outpushings in digestive tract. A, 6 mm. pig 
embryo; B, same at 8 mm.; C, same at 10 mm. t, trachea; e, oesophagus; s, 
stomach; /, liver; d.p., dorsal pancreas; v. p., ventral pancreas; s.i., small intestine; 
l.i., large intestine; c, caecum; v.d., vitelline duct. (From Carey, Journal of General 
Physiology. Vol. III. No. 1.) 

II. Three schematic views of variations in the ducts leading from the gall- 
bladder, c and s, cystic duct; ch, ductus choledochus; h, hepatic duct; he, hepato- 
cystic duct; he, hepato-enteric duct; vf, gall-bladder. (From Schimkewitsch after 

III. A diagrammatic section of the cloaca of a male bird. (After Gadow.) 
cd., Upper region of cloaca into which rectum opens; ud., median region into which 
ureter (m.) and vas deferens (vd.) open from each side; pd., posterior region into 
which the bursa Fabricii (B.F.) opens. 

ings (Fig. 426) of this elementary digestive tract. It will be necessary 
to remember in one's study of all the higher forms, that no matter how 
many of these inpushings or outpushings there may develop, and no 
matter how lengthy the digestive tube may grow and coil, if it be 
straightened out, it will to all intents and purposes be a continuous hol- 
low tube. The interior of this hollow tube is really outside the body 
in so far as it is subject to all the external conditions to which the body 
itself is subject. In other words, one may the better understand this 
if a hollow gas pipe, open at both ends, is thought of. The hollow 
straight opening, through which the eye can see, represents the digestive 
canal. The metal of which the pipe is composed represents the walls 
of the digestive tube. It can, therefore, easily be seen that any con- 

300 Comparative Anatomy 

ditions, such as dust and moisture, that may be in the atmosphere sur- 
rounding the outside of this pipe, will quite likely be found on the 
inside also. 

During the development of the nervous system there is a connection 
between the lumen of the neural tube and the gastrular mouth so that 
there is a temporary connection between the neural tube and the gas- 
trocoele. This connection is called the neurenteric canal (Fig. 328, B, 
ne.c). This connection, however, soon disappears so that the gastrocoele 
is a closed sac with no opening whatever to the outside of the body until 
the mouth and anal openings are pushed in from the ectoderm as already 

Definite names are given to the various structures of the growing 
embryo. The mouth opening is called the stomatodeum. The mid por- 
tion connecting the mouth with the anal opening is called the mesodeum, 
and the caudal part, which like the stomatodeum is ectodermic, is known 
as the proctodeum. 

This does not mean that, in every animal in which an ectodermal 
mouth and anus has developed, the ectodermal structures take up the 
same length of the digestive system. In the articulates (crustaceans, 
insects, and spiders) the stomatodeum and proctodeum are much longer 
and larger proportionately to the mesodeum than in the higher forms of 
life ; in fact, in the vertebrates, the digestive canal is mainly mesodeal 
and, therefore, endodermic. The mouth and anal regions composed of 
ectoderm are but a small portion of the entire digestive system. 

The jaws, teeth, and tongue, which will be taken up separately, do 
not develop from the simple digestive tube which has just been de- 
scribed; but the other parts of the digestive system, even the most 
complicated ones, have come from this tube alone by a growing in 
length, by enlargements of various kinds, by foldings, by outpushings, 
and by inpushings. Not only have such complex organs as the liver 
and spleen, thyroid and thymus glands, as well as many others, come 
from this endodermal tube, but the entire breathing apparatus of 
chordates has arisen from its cephalic end. 

As some chordates, such as fishes, live in water, they require a 
totally different type of breathing mechanism than those which live on 
land. Still their branchial, or gill, system and the land living pulmonary 
or lung system have in each case developed from the same simple diges- 
tive tube. It must be remembered that this is only true of chordates. 
Animals, not chordates, do not show such close relationship between the 
digestive and respiratory systems. 

The intestinal tract, if cut in cross section and examined micro- 
scopically, will be found to consist of four layers of different types of 

Digestive System ^ 301 

cells. Starting from the inner layer we find them in the following order 
(Fig. 291) : 



( circular 
muscularia •^, . ,. , 
( longitudmal 


It is in the mucosa or the inner layer that the glands which produce 
the digestive juices are found. Here, too, are a few scattered involuntary 
muscle fibers and lymphatic vessels for carrying away the nutriment 
after it has been changed into a condition so that it can be assimilated. 

The submucosa is a thin layer of connective tissue supporting the 

The muscular layer varies a good deal, but essentially it is composed 
of two layers of involuntary muscle cells both circular and longitudinal, 
the former lying toward the lumen. 

The circular muscles, by contracting, lengthen the intestines, while 
the contraction of the longitudinal muscles shorten and thicken them. 
These two actions cause the peristaltic movement which occurs during 
digestion, pushing the food forward and also permitting the various 
folds and little finger-like projections, called villi, to come in contact 
with all of the material that has been ingested. 

In the higher forms, especially in the human being, there are some 
thirty feet or more of small intestine as contrasted with three or four feet 
of large. The reason for this can be understood quite readily when it is 
appreciated that a two-inch water pipe holds four times as much as a 
pipe one inch in diameter. The great mass of material that is ingested 
is of no value whatever to the animal ingesting it unless such food can 
be reduced to a more or less liquid state and be absorbed by the mucous 
lining of the intestinal tract. Digestion, though beginning in the stom- 
ach, really takes place in the small intestine. The smaller this intestine 
is in diameter, therefore, and the more folds the mucosa has, the more 
readily will the food, after it is sent through the digestive canal, be 
likely to come in contact with the mucosa and be absorbed. 

It is important to understand this as it will throw much light upon 
various physiological functions of digestion, for, it will be seen that the 
little finger-like villi must actually do the work of absorbing. That is, 
after ingested material is ready for absorption, it does not pass by any 
rule of gravity or mechanics into any definite opening; but these little 
projections must actually reach out and drink in the necessary material. 
As these little villi must in turn be kept in good condition and capable of 
performing their functions by their nerve and blood supplies, it follows 
that, where the nerve and blood supply is either weakened or lost, the 
animal may die of starvation regardless of how much food it may ingest. 


Comparative Anatomy 

Many glands are found in the mucosa. Some of the larger ones have 
pushed their way further and further back so that they have not only 
passed through the submucosa and muscularia, but have gone far beyond. 
The liver and pancreas are good examples of those which have left the 

lytic f^lof y^ytet. 
PefTal i/e«>» 


Fig. 427. 

I. Diagrams to sbow formation of greater omentum in mammals and the 
fusion of the mesogaster and the mesocolon. A, early stage in which the mesogaster 
is beginning to form a bag at g. B, the mesogaster is drawn posteriorly into a 

Digestive System 303 

main digestive tract almost entirely but are still connected v^ith it by 
small ducts. 

The glands push their v^ay through both submucosa and muscularia 
but, as they push against the serosa, this seems to stretch out ahead of 
all these outpushings to form a covering for the outgrowths. This is 
why, not only the liver and pancreas, but every organ in the abdominal 
cavity is completely covered by this serous layer, which, when thought 
of in its entirety, is called the peritoneum. 

The kidneys form a single exception to the statement that all organs 
in the abdominal cavity are completely covered by peritoneum. These 
do not spring from the digestive tract, however, and will be discussed 
later with the uro-genital system. 

The entire digestive canal is covered with this serous layer. Figure 
427 shows just how this develops and why it is that, while there is a 
single layer of serosa over the ventral side of the intestinal tract, there 
are two layers running dorsalward which are attached close to the ven- 
tral portion of the spinal region to form the sustaining ligaments. 

Probably this may be made more understandable if the student 
places an ordinary sheet of paper on the desk before him and lays a 
pencil at right angles to the long axis of the sheet. By picking up the 
two ends of the paper so that the pencil is held within the fold, it will 
be seen that under the pencil there is only one layer of paper but above 
it there are two. The various outpushings of the intestinal tract push 
the serosa before them just as the pencil does the paper in this case. 

The two layers running dorsalward from the organ to form the 
sustaining ligament, are called the mesentery. It is between the two 
sheets of mesentery that the blood supply of the organ is carried. 

If it be remembered that the digestive tract begins as a single tube, 
approximately the same length as that of the body in which it grows, 
and if the various elongations, outpushings, and inpushings are then 
followed through the embryonic period, considerable light will be thrown 
upon our understanding of the adult structure (Fig. 428). 

One must, however, be wary in comparing diiTerent type-forms of 
animals, as well as animals of the same species at different stages of their 
development, or there will be little validity in the comparisons. 

The first portions of the digestive tract to differentiate are the 

long bag g which is the greater omentum; the mesogaster and mesocolon are fusing 
at i. C, completion of the fusion of mesogaster and mesocolon at i. a, liver; h, 
serosa of the liver; c, lesser omentum or gastro-hepato-duodenal ligament; d, 
stomach; e, lesser peritoneal sac or cavity of the greater omentum; /, mesocolon; 
g, portion of the mesogaster which forms the greater omentum; h, intestine; i, 
fusion of the mesogaster and mesocolon. (From Hyman after Ilertwig.) 

IT. Scheme of digestive canal and mesenteries in human embryos, 30 and 50 
mm. long, ac, ascending colon; c, caecum; co, colon; d, duodenum; dc, descending 
colon; k, kidney; r, rectum; rd, recto-duodenal ligament; rl, recto-leinal ligament; 
rrd, recto duodenal recess; s, stomach; sp, spleen; tc, transverse colon. (From 
Kingsley after Klaatsh.) 

III. Transverse section of a salamander embryo in the region of the liver. 
(Redrawn from Maurer.) 

IV. Schematic arrangement to show the development of the omental bursa. 
(After Corning.) P, Pancreas; Ao, Aorta; L.H.G., Hepatogastric ligament. 


Comparative Anatomy 






Fig. 428. 

Reconstruction of the diges- 
tive canal of man. al, allantoic 
stalk; cl, cloaca; g, glottis; h, 
hyoid arch; li, liver; lu, lung; 
md, nix, mandibular and maxil- 
lary arches; n, nasal pit; o, 
omphalomesenteric vein; s, 
stomach; v, visceral arches vi, 
vitelline stalk; w, Wolffian body. 
(From Kingsley after His.) 

pharynx and stomach. The former is a fun- 
nel-shaped enlargement at the cephalic end 
with several pairs of lateral diverticula called 
the pharyngeal pouches. These pouches in 
some animals break through to the outside 
of the body to form slits (Fig. 295). The 
stomach may be of many shapes and sizes 
in the various animals. That portion of the 
stomach which meets with the oesophagus (the 
narrow tube connecting pharynx and stomach) 
is known as the cardiac portion, while the 
caudal opening of the stomach is called the 
pylorus (Fig. 438). It will be found that this 
pyloric end is rather thick and tough. There 
is a valve here which closes so that the stom- 
ach can be converted into a closed sac. A 
rather thick short portion of the intestine im- 
mediately caudal to the pylorus is known as 
the duodenum. Then follows the small intes- 
tine, varying in length in all the animals. It 
ends in the large intestine, and this in turn 
connects directly with the anal opening to the 
exterior of the body or in a terminal enlarge- 
ment which quite often receives the openings 
of the urinary and reproductive systems before 
connecting with the anal opening. In the lat- 
ter case, the thickened portion of the large 
intestine is called the cloaca! chamber or, sim- 
ply, the cloaca. (Fig. 426, III.) 

In fishes, amphibians, and sauropsida, the 
cloaca is an important structure. In none of 
the mammals, except the monotremes, does it 
appear as a distinct organ, 
diverticula of various shapes are thrown out 
The lateral pharyngeal pouches have already 

Various important 
along the digestive tract, 
been mentioned. In fishes one often finds quite numerous pyloric caeca. 
In mammals at the beginning of the large intestine, where the small one 
enters it, there are colic caeca. In man as well as in several other forms 
of mammals, one of these little blind sacs is called the appendix vermi- 
formis. In birds one finds cloaca! caeca. 


The pharynx is that open portion behind the nose and mouth in 
mammals which extends down to the voice box. From there downward 
(including the voice box) the open portion is called the larynx. 

There are two general types of mouth forms. The first is found in 

Digestive System 305 

the great group of Cyclostomata (cycle mouths). There are no true 
jaws (Fig. 422). The mouth is round and cannot be closed. Examples 
of this form are the lampreys and hagfishes. This type of mouth is 
called suctorial. The cyclostomata are the only vertebrate parasites 
known. They attach themselves to a living fish and suck their way 
directly through the muscles of the host. 

The second type of mouth belongs to that group called the Gnathos- 
tomata (jaw-mouths). This type of animal has movable bones or car- 
tilaginous jaws and usually possesses teeth formed of dentine and under- 
laid with enamel. The jaws are developed from one pair of visceral 
arches. The teeth are quite similar to the placoid scales of certain fishes 
which have been modified in various ways. There are two theories held 
in regard to the Gnathostomata or jaw-mouth fishes. First, that the 
mouth is like that of the cyclostomes, to which the gill arches with their 
associated teeth have been added; and the second, that this jaw-mouth 
is a new opening which originally consisted of a pair of gill-slits which 
later became fused in the mid ventral line, the first mouth then being 
lost. Probably the latter view has more supporters, because in the 
selachians, where there is supposed to be a more primitive condition, 
the jaw-mouth is not at the extreme cephalic end of the animal, but on 
the ventral side with a long rostrum extending cephalad to it. Later, 
in some of the ganoids, this jaw-mouth has a secondary position at the 
very tip or terminal end. 

The pharyngeal pockets develop from a row of outpushings meeting 
a similar set of inpushings from- the outside (Fig. 295). If the point of 
contact is broken through, as in fishes, such openings are called gill-slits. 
These are four to eight in number, which permanently form a communi- 
cation between the pharynx and the exterior to allow the escape of water 
taken in by the mouth for use in breathing. 

One or more of these slits appear in the early stages of amphibians 
and in a few forms persist throughout life. In reptiles, birds, and mam- 
mals, there are similar inpushings and outpushings during the embryonic 
period, but only two or three contact-points ever form openings, and 
then only for a short time. However, the most anterior of these which 
appears in the selachians as the spiraculum or blow-hole, persists in all 
higher vertebrates as the Eustachian tube and the greater part of the 
middle ear. The other pouches disappear, although cartilages, muscles, 
arteries, and glands arise in the embryo in connection with these 

Sometimes there is an arrested development so that an open com- 
munication persists between the pharynx and the exterior of the jaw 
either upon one or both sides. This is called a cervical fistula. It is 
supposed to be a permanent gill-slit that for some mechanical or chemical 
reason did not continue growing as it normally should have done. 

The nasal cavities in fishes lie above the stomato-pharyngeal cavity 

306 Comparative Anatomy 

and are unconnected therewith, while in amphibians (Fig.. 339) there 
develops a pair of openings called the posterior-nares, or choanae, con- 
necting these two. portions by openings in the roof of the mouth. This 
communication is supposed to be one of the changes which was brought 
about during the transition from a life of water to a life on land. It 
allows the ingress and egress of air to the pharynx and then to the lungs 
without opening the mouth. This action, although harmless for an ani- 
mal living in water, would soon cause the drying up of the mucous mem- 
brane lining the mouth cavity if resorted to in air with anywhere near 
the same frequency. In the nasal cavities this is prevented at least in 
part by the small size of the external openings, but still more by the 
formation of slime glands which produce considerable secretion. Then, 
too, the waste lacrimal fluid diverted from the eyes to the nose is, in all 
probability, also of assistance in this respect. 

There is a tendency in the pharynx to form diverticula in the median 
line (Fig. 426), that is, there is here an expansion into large sacs or 
reservoirs which may, or may not, remain in communication with the 
pharynx itself. The air-bladders in fish are examples (Fig. 441). While 
this air-bladder is usually a closed sac filled by gases extracted from 
the blood, there are a few animals in which one finds a rather small 
air-duct passing from these air-bladders to the pharynx. In fishes, where 
this occurs, the animal comes to the surface of the water and makes a 
snapping or swallowing movement. In the higher forms of animals this 
develops into the pulmonary system, the lateral sacs being the lungs 
and bronchia and the median duct, the trachea. The opening of the 
trachea into the pharynx is called the glottis (Fig. 428), which together 
with the various cartilages and muscles derived from the visceral system, 
forms the larynx. While there are always two lungs in lung-breathing 
animals, there is only one air-bladder in those forms of vertebrates which 
are not lung breathing. Then, too, the air-sac, or air-bladder, is almost 
always dorsal to the pharynx while the lungs lie ventral to it. 

The flat plate of bone which forms the roof of the mouth and thus 
separates the nasal cavities from it, is called the hard palate. The soft 
cover of this bony plate which extends backward beyond the palate is 
known as the soft palate or velum palati. 

Just as one can easily see the sulcus in the median line imme- 
diately under the nose, so, in looking into the mouth, a ridge will 
be seen, which is formed right through the center of its roof. As the 
human being, as well as all of the higher forms of animals, is bilat- 
erally symmetrical, and as different portions of the jaw begin their 
growth from distinctly separate centers, which then grow toward the 
midline and unite, one can readily understand not only why a ridge is 
formed on the roof of the mouth but also why there is a sulcus immedi- 
ately below the nose on the outer upper lip. If, due to a mechanical or 
chemical obstruction of some kind, the two lateral portions of palate or 

Digestive System 



-"y^'T^ ■ 

Denial lam- 
ina . 
Genu jcr per- 
niancnt tootli. 


Germ for per- 
mayient tooth. 

^^ Enamel. 

Enamel cells. 


Y i 

Fig. 429. 
I. II, III, IV. Diagrams of developing 
tooth. (After Hill.) 
V, Section through the skin of an Elas- 

upper lip do not meet, a harelip and 
cleft palate result. 

In some birds the two halves 
of the palate never unite. In some 
mammals, such as the cat and dog, 
the two portions forming the upper 
lip have not united as well as they 
have in the human being, and, con- 
sequently, a deep median groove 
called the philtrum, remains. This 
line can be seen to run along the 
entire septum of the nose exter- 

Next in order of study come the 
teeth, tongue, tonsils, glands of the 
mouth cavity, and glands of the 
pharyngeal pockets. 

There are two types of teeth 
which have no relationship to each 
other in their origin. The true 
teeth are akin to placoid scales 
(Fig. 429). They arise by a calcar- 
eous secretion at the junction where 
ectoderm and mesenchyme meet 
and are thus a product of both 

The other type comes purely 
and simply from the cuticle and is 
formed by what is known as cornifi- 
cation or hardening of the epi- 
thelium (Fig. 422, 6). The parts 
which have invaginated to form the 
stomatodeum retain the capacity to 
form hard structures; consequently, 
any portion of the mouth-walls may 
secrete scale substances. It is nec- 
essary to appreciate this in order 
to understand that in the different 
type of fish and amphibia, teeth of 
almost any number, size, and shape 

mobranch sl-owinsr formation of a dermal 
spine. Hig'ily magnified. 

1. ITorny layer of ectoderm. 2. Mal- 
pigVian layer. 3. Columnar cells of ecto- 
derm secreting. 4. Enamel. 5. Dentine 
(black). 6. Dentinal pulp. 7. Connective 
tissue. (From Shipley and MacBride.) 

308 Comparative Anatomy 

may be found wherever there is cartilage, or bone, to hold them. In the 
higher forms of animals, in fact, in all the amniotes, v^ith the exception 
of some squamata, teeth are found only on the margin of the jaws. 
Turtles and all present forms of birds are toothless, though many extinct 
birds, of which fossil remains have been found, did have teeth. It is 
interesting to note that even in turtles and birds that have no teeth, the 
dental ridge in which the teeth of toothed animals do develop, is never- 
theless present in the embryonic stages, it being assumed that this is 
proof of their descent from toothed ancestors. 

It will be observed in Figure 429 that at first the ectoderm thickens. 
The layer of ectodermal cells pushes downward into the mesenchyme. 
The mesenchymal cells, then, by multiplying rapidly, push portions 
of this ingrowing plate of cells back up and form a sort of finger-like 
projection covered by the plate it has pushed before it. The mesen- 
chymal finger-like projection forms the pulp of the tooth, while the plate 
of cells which covers it becomes what is known as the enamel organ. 
The pulp forms several layers of cells, the outer ones becoming odonto- 
blasts, so called because it is from these that the bone-layer-substance 
dentine or ivory of the tooth is formed. This latter substance is a secre- 
tion from the ends of the odontoblasts and it is this which causes it to 
be somewhat prismatic in form. 

At the base of the enamel organ a denser substance, called enamel, 
is secreted. This fits like a cap over the top and sides of the dentine. 
The dentine continues to grow and forces the tooth up through the 
epithelium so that the tip, or crown, then comes into position for use. 
The nerve supply of the tooth comes from branches of the trigeminal or 
fifth cranial nerve. Both nerves and blood vessels enter through the 
base of the tooth. Usually, as soon as the teeth are fully formed, the 
odontoblasts cease growing. However, there are exceptions to this 
rule. The tusks of elephants and the incisors of rodents function through 
life and, therefore, continue to grow. 

In mammals an additional layer of modified bone, the cement, is 
formed around the root of the tooth. It may even extend through the 
crown. The teeth in the mouths of skates and some other elasmobranchs 
are arranged very much like the scales on the surface of the jaw, that is, 
in groups of five. In most of the vertebrates there is a succession of 

Some animals, such as the shark and turtle, continue to renew and 
shed their teeth. Such teeth are called deciduous. 

In mammals, a second set of teeth usually arises directly behind or 
above an.d below the first set, so that the ends of the second set, which 
are to force their way through the jaw, push against the roots of the 
teeth which are already in use. 

The group of first teeth formed in man is called the milk dentition. 

Digestive System 


The second is known as the permanent dentition. In some mammals, 
such as the monotremes, sirenians, and cetacea, there is only one 
dentition; while in some groups there are an indefinite number of suc- 
cessive dentitions. In such animals as guinea pigs, and in some bats, 
the milk dentition is lost even before birth. 

Practically all fishes with few exceptions have teeth, and these 
extend not only to the lining bones of the mouth but, in some, even to the 
hyoid and branchial arches. These latter are known as pharyngeal teeth. 

Biting mechanism of the rattlesnake. la, and lb, position of the ap- 
paratus when mouth is shut. IJa, and lib, position of the apparatus when 
mouth is opened widely, showing the spheno-pterygoid muscle (P.e.) con- 
tracted, the pterygoid (Pt) pulled forward, the transverse bone or 
ectopterygoid (Tr) pushing the maxillary (M) rotating it and thereby 
causing the poison-fang (J) to assume an upright position. Di, Digastric 
muscle, the contraction of which lowers, or opens the lower jaw; G, the 
groove or pit characteristic of the Crotaline snakes; /, poison fang; M, 
maxillary; P, palatine; P.e, spheno-pterygoid; Pm, preraaxillary; Pt, 
pterygoid; Q, quadrate; Sq, squamosal; T, a, insertion of the an- 
terior temporal muscle, by contraction of which the mouth is shut; Tr, 
transversum or ectopterygoid; X, origin and insertion of a muscle and a 
strong ligament, contraction of which draws the maxillary and its tooth 
back into the position of rest and assists in shutting the mouth. (After 

The teeth may be cone shaped or flat, sometimes they even form large 
plates as though a number of primitive teeth had grown together. Teeth 
may be anchylosed to the summit of the jaws, attached to their inner 
side, or have their roots implanted in grooves or pockets as in the human 
being. The grooves in the jaw, in which teeth grow, are called alveoli.^ 

^Mammals are said to be monophyodont if they develop only one set of teeth, and diphyodont if 
they develop two. However, even in monophyodont mammals, a second set usually develops, although 
this set later becomes absorbed or remains in a vestigial condition. 

When all the teeth are uniform they are said to be homodont, while if they vary in shape they 
are heterodont. 

Teeth are said to be acrodont, if anchylosed to the summit of the jaws, pleurodont, if fastened 
to the jaw's inner side, and thecodont, if the roots are implanted in alveoli. 

Teeth have also recejved najnes according to their function or their peculiar physical appear- 


Comparative Anatomy 

The poison fangs of certain serpents are really specialized teeth on 
the maxillary bones. They may be permanently erect or turn as on a 
pivot so that when the mouth is closed the teeth lie along the roof of 
the mouth. Vipers and rattlesnakes are examples of this latter type. 
(Fig. 430.) 

Fig. 431. 

The types shown have been chosen from the principal families of the Carnivora in such a 
manner as to present a complete view of the changes in dentition in that order. They are: 

P, Proviverra (.Cynohyaenodon) Cayluxi {Creodonts) ; V, Viverra {Civets) (Viverridae) ; H, 
Hyaena crocuta L, {Hyaenidae) ; F, Felis leo L. (lion) (Felides) ; M, Michairodus cultridens Cuvier 
(Ancient saber-toothed tiger of the Tertiary age) (Felides); C, Canis familiaris L. (common dogs) 
(Canides); U, Urstis arctos, L. (Bears) {tjrsides). 

Letters used in common for the figures of the groups of teeth: i, incisors; c, canines; />,_ pre- 
molars; m, molars; k, carnivores; cm, inferior maxillary condyle; gl, glenoid fossa; co, occipital 

I. CREODONTS. Extinct order of the Eocene and of the lower Oligocene; supposed to be 
the common stem of all the carnivora. 

Type form: Proviverra (Cynohyaenodon) Cayluxi Filhol, of the phosphorite of Quercy (upper 
Eocene and lower Oligocene). 

P — Right half of the base of the skull seen from below. Below, fragment of the right half of 
the lower jaw, internal aspect. 

3 14 3 

Complete Dental Formula — — 

3 14 3 

ance. For_ example, they are said to be secodont, if used for cutting purposes, such as those of cats; 
hunodont, if used for crushing as in man; lophodont, if they possess well-marked transverse ridges 
as in the elephant; and selenodont when they possess longitudinal crescent-shaped crests as in the 

Digestive System 311 

Premolars becoming complicated from before backward, to pointed tuberosities, and compressed 
laterally (secodont type); the fouith premolar ip_,) and the three upper molars united to three 
tuberosities by sharp ridges. On the lower jaw, these teeth present the entire anterior surface to 
three pointed tubercles and a flattened posterior heel (Type: tuberculo-sectorial of Cope). 

II. CARNIVORA. They are distinguished from the Creodonts, from the point of view of 
dentition, by the differentiation in the two jaws, of a carnassial, or tooth of slicing action, made 
apparent by its greater development than that of the other molars. It is the 4th premolar of the 
upper jaw; in the lower jaw it is the 1st molar. 

In a general manner, all the teeth placed before the carnassial, that is to say all the premolars, 
are sharp-pointed; all those which are behind it are tuberculated. 

1. Viverrides {Civets) : The most primitive of the Carnivora properly speaking (true Oligo- 
cene), from which all the other forms are usually considered to have been derived. Type shown: 
Viverra indica Desm. 

V, skull, seen from the right side. The maxillae have been dissected away sufficiently, as m 
the other figures, so as to show the roots of the teeth. 

Vs, left superior carnassial tooth, seen from the crown. 
Vi, left inferior carnassial, seen from the internal aspect. 

3 1 4 (3) 2 
Dental Formula 

3 1 4 (3) 2 
The number of molars is in general reduced to two, characteristic of a carnivorous specialization. 

2. The Mustelids (marten, sable, polecat, weasel, stoat) are very close to the primitive type. 
The carnivorous tendency is strongly developed, as shown by the great reduction of the molars and 
the higher development of the carnassial tooth. 

Starting from the Viverrides. the various forms of the carnivora show changes in two clearly 
divergent directions: one, in which the meat-eating nature of the animal becomes more and more 
evident, as in Hyaenidae, Felides, and Pinnipedes (seals, eared seals, walrusses) and the other which 
returns somewhat to the omnivorous order, separated from the Hyaenidae and Felides, and giving 
rise to the Canides and Ursides. 

3. Hyoenides: These form a branch supposedly derived in a direct line from the Viverrides 
(Hyaenictis) as they appear in the upper Miocene, 

Type Figure: Hyaena Cr acuta L. 

H, the two jaws, seen from the left side. 

Hs, left superior carnassial tooth, seen from the crown. 

Hi, left inferior carnassial tooth, internal view. 

3 14 1 
Dental Formula ■ — 

3 13 1 
Dentition quite like that of felides, and not well developed in a carnivorous sense. 

4. Felides (cats) : The most characteristic of the Carnivora. Their most typical representa- 
tives appear in the Miocene type, but they are preceded by others, which connect them with the 

Type Figure: The lion (Felis Leo L.) ; Machairodus cultridens Cuvier, (a fossil Feline of the 
European Pliocene age). 

F,, Skull of a lion, seen from the left side. 

F^, Left half of the same skull, seen from below. 

F^, Left superior carnassial, seen from the crown. 

Fi, Left inferior carnassial, seen from the internal aspect. 
3 13 1 

Dental Formula 

3 12 1 

Of the tuberculated molars, a single one persists, very much reduced (w^). The premolars, 
although secondont, have undergone a certain reduction in their number as well as in their size, 
leaving all the functional importance to the carnassials, which have become enormous. The canines 
are_ likewise very strong, and are much longer than their neighbors. On the other hand, the 
incisors, whose cutting function is done much more efficiently by the carnassials, have diminished. 
The jaw is, all in all, greatly shortened. Notice also the great development and widening of the 
zygomatic arch, giving a large surface for the levator muscles of the lower jaw (the temporalis, 
which passes under the arch, and the masseter, which takes its origin from the entire length of the 
arch). It has thus acquired considerable size and strength. The lower jaw is greatly hollowed out 
on its external aspect, to permit insertion to the fibers of the large masseter muscle. 

M, Skull of Machairodus cultridens (extinct saber-toothed tigers), seen from the left side. 

Ms, left superior carnassial, seen from the crown. 

An exaggeration of the Feline type. 
3 12 

Dental Formula 

3 111 
Huge development of the superior canine teeth, which surpass so far those of the lower jaw that 
they limit closely, on each side, the buccal gap, no longer permitting free use of the canines and 
carnassials in tearing off meat. 

5. Canides (dog-like carnivora). A mixed group, both meat-eating and omnivorous. The 
canides appear early in the Oligocene, their first forms being closely related to the primitive Viverra 
or to the Creodonts, some of whose characteristics are even more primitive than those of the typical 
Viverrides. The dog family appears in the early Pliocene. 

Type Figure: Canis familiaris, L, 
C, Left half of the skull, seen from below. 

C', Left inferior maxillary condyle, seen as a horizontal cylinder (characteristic of all the 
Carnivora) in relationship with the glenoid cavity which is hollowed out cylindrically. 
C,, The skull, seen from the right side. 

312 Comparative Anatomy 

Cs, Left superior carnassial, seen from the crown. 

Ci, Left inferior carnassial, seen from the internal aspect. 
3 1 4 3 (2) 

Dental Formula 

3 13 3 

6. Ursides (Bears). The least carnivorous of all the Carnivora. They originated, apparently, 
in the upper Miocene, from the primitive Canides (Amphicyon). 

Type Figure: Ursus arctos L. 

U^, The two jaws seen from the right side. 

U^, Left half of the base of the skull seen from the lower aspect. 

This figure has been placed near the corresponding figure of the Lion in such a manner as to 
render apparent the comparison between these two extreme types. The comparison must be limited 
to the portion included in each figure between the incisive teeth and the occipital condyle (co). 
Behind this there is a very large hollowed out area which projects from the posterior aspect, to serve 
as the insertion of the posterior muscles of the skull and neck. 

Us, Left superior carnassial, seen from the crown. 

Ui, Left inferior carnassial, seen from the internal aspect. 
3 13 2 

Dental Formula ■ — 

3 12 3 
This is probably a regressive adaption to the omnivorous regime. The enormous development of the 
molars have become quadrituberculated and complicated by the appearance of little tubercles on or 
between the greater tubercles. Regression of the cutting function of the teeth has followed, the flesh- 
eating character becoming hardly apparent except in the remarkable power of the upper canines, with 
their very oblique insertion and long root. The skull also lengthens. 

Changes which take place in the Superior Carnassial Teeth: (Figs. Vs, Hs, Fs, Ms, Cs, Us. 
In the figures the arrow indicates the upper teeth, the arrow's point being directed toward the 
opening of the mouth.) 

a, paracone or antero-external cusp; c', anterior accessory cusp; b, protocone, or antero-internal 
cusp; c, metacone, or postero-external cusp. 

Primitive form (Viverra, Vs); type trigodont (triconodont). The tooth contains two external 
cusps (a,c) compressed laterally and united in a single cutting edge, and a third tubercle (&) placed 
anterior and forward. A fourth tubercle (a') is often found in front of the two external cusps 
on the same line with them. The cutting edge formed by these last is lengthened in such a manner 
as to place these tubercles together and thus present three points. The tooth has three roots, two 
anterior and one posterior. _ _ 

Hyaenidae and Felides (Hs and Fs) : _ The external cutting edge is developed highly in these. 
The internal anterior tubercle remains conical and blunt, but disappears completely in Machairodus 

Canides: (Cs) : Changes in the omnivorous group. The tubercle a' has disappeared, the 
tubercle b remaining prominent. 

Ursides: (Us): There is the same type of accentuation as in the Canides. Three conical 

Changes in the Lower Carnassial: (Figs. Ui, Ci, Hi, Fi, Vi). ,The arrows are placed above the 
tooth to indicate the lower ones. The point indicates the anterior direction, a, paraconid or anterio- 
internal cusp; B, protoconid or external-anterior cusp; B', metaconid or internal posterior cusp; Y, 
hypoconid, or posterior talon. 

The primitive type (Viverrides, Vi) is here the tuberculo-sectorial type of Cope. It contains 
(1), An anterior part with three tuberculated points; two internal, A, jB', and one external, B, (2) a 
posterior talon Y, low and flattened, carrying one or more blunt tubercles. The tooth presents, in 
other words, a secodont anterior portion (carnivorous) and a tuberculated posterior portion (omniv- 
orous). It has two roots corresponding to the two parts. 

This tuberculo-sectorial type is common to all the lower molars of the creodonts and is limited 
more or less to the true carnassial tooth in the true Carnivora. 

Canides. Ci: The tuberculo-sectorial type is preserved but with accentuation of the carnassial 
character. Then there is a reduction of the talon predominance of B, reduction of B' , and the 
anterior root is somewhat stronger than the posterior. 

Hyaenidae, Hi: Regression of the talon; the tubercles A and B compressed and united in a 
sharp cutting edge, and bicuspid; 5' notably reduced. The anterior root is much stronger than the 

Felides, Fi. : The talon of B' has nearly disappeared and there is predominance of the anterior 

Ursides, Ui.: Omnivorous type; the secodont part is smooth; its tubercles are blunt and conical; 
the talon contains more than half of the crown; it is covered by the secondary tubercles, which are 
elevated almost to the level of the anterior cusps. ^ The whole of the talon has this one surTace 
entirely similar to that of the tuberculated molars which are placed next in position. 

Title Figures: P, head of Panther (FELIS); Ci, head of Civet (VIVERRA). (From the 
charts of Remy, Perrier & Cepede.) 

There are four kinds of teeth in mammals (Fig. 431). In the human 
being, they are alike in both upper and lower jaws as well as alike in 
both halves of upper and lower jaws. For classification of teeth, we use 
only one-half of the teeth in either jaw. Thus in man, we find the two 
teeth nearest the midline — the incisors — are followed by a single canine. 
This is distinctly cone shaped and has a single root. Back of this come 
the two pre-molars, commonly called bicuspids, having two roots and 
complicated crowns. They appear both in the milk and permanent den- 

Digestive System 313 

titions; and lastly three molars quite like the pre-molars in form, with 
several roots, but appearing only in the permanent dentition. The num- 
ber and kind of teeth is expressed by v^hat is known as a dental formula. 
As already stated, the number and kinds of teeth in the two halves of the 
jaw are the same, so only one side need be represented in the formula, 
but, as in some animals the upper and lower jaws do not have the same 
types and forms of teeth, the formula must take both upper and lower 
jaws into consideration. The upper figures, therefore, represent one-half 
the upper jaw and the lower figures one-half the lower jaw. 


i-r, ^"T, pm-|-,niT' 

The foregoing is the dental formula for man ; that for the opossum 
is as follows : 

It , c— , pm— , ni— 


Epidermal teeth occur in (Fig. 422, 6) and various 
larval stages of amphibia and monotremes. In the cyclostome these are 
little cone-like projections of cornified epithelium with an underlying 
core of integument. These epidermal teeth are differently arranged in 
the lampreys and myxinoids. In the myxinoids they are few, only a 
single tooth being found on the palate and two chevron-shaped rows on 
the top. In the lampreys almost the whole inner surface of the oral hood 
is lined with these teeth of varying shape and there are a varying num- 
ber upon the tongue. Epidermal teeth are used as a means of fastening 
the animals to their prey. Those on the myxinoid tongue are used for 
boring into the fishes on which these parasites feed. In the larval anura 
(Fig. 318) there are cornified papillae serving as teeth along the edge 
of the jaws. The arrangement varies in different genera. They are 
frequently aggregated in dental plates which the animal uses in scraping 
the algae from submerged objects. They are not related to the teeth 
of cyclostomes. 

Baleen, or whalebone, should be mentioned here. This is formed in 
large plates of horny material attached to the margins of the upper jaw 
(Fig. 392). The ffinged ends and edges of these plates serve as strainers 
to extract the food products from the various materials taken in with 
the water. 

In the embryos of certain lizards and snakes there is a median tooth 
which projects from the mouth and which is used to rupture the egg 
cell when the young is ready to escape. Such a tooth is called an egg- 
tooth. An egg-tooth is formed in turtles, Sphenodon, crocodiles, birds, 
and monotremes, but in these cases it is only a thick (sometimes cal- 
cified) portion of the epidermis, 

314 Comparative Anatomy 


The tongue varies to a very considerable extent in the different 
groups of vertebrates (Fig. 432). In mammals the hyo-branchial sup- 
port consists simply of a basi-hyal (body) and tw^o pairs of horns 
(cornua). The most cephalad pair are the longer and usually consist of 
four bony structures, the cerato-hyal, the epi-hyal, the stylo-hyal, and 
the tympano-hyal, the latter bone attached to the skull in the tympanic 
region. The pair of horns lying caudal consists of only a single skeletal 
piece of bone known as the thyro-hyal which connects the body with the 
thyroid cartilage of the larynx. In the human being the anterior, or 
cephalad, horns are considerably modified from those in other mammals. 

The tympano-hyal and the stylo-hj^al have fused with the otic region 
of the skull to form the styloid process, while the hypo-hyal is a mere 
rudiment connecting with the styloid process by a ligament; the cerato- 
hyal is not present. The anterior horns, though typically longer and 
m.ore complex than the others, are called the "lesser" in man, because 
the earlier anatomists took all of their names from the human being 
without any comparisons with other forms. 

There is no functional tongue in fishes, although the material which 
develops into a tongue in the higher forms is present. This is known as 
the anterior part of the hyo-branchial apparatus. The more cephalic 
part of this complex apparatus is found in the floor of the mouth cavity. 
It is naturally shaped according to the jaw outlines which border it. 
It may even be pushed forward so as to form a slight elevation by the 
action of the visceral muscles. 

In amphibians, where the gill-bearing function has more or less 
ceased, this region forms the basis o£ the tongue while a fleshy organ of 
some kind may develop. 

In the higher forms of vertebrates two to four of the visceral arches 
form the skeletal basis of the tongue. The hyoid arch is the structural 
foundation to which the muscles o£ the tongue are attached. Here one 
usually finds, although there are many varieties, a median basi-branchial 
piece, called the os entoglossum, and two caudad projecting horns — the 
cornua. In Sauropsida the tongue is a direct condition of this and the 
principal motion consists in protruding and withdrawing the entire 
organ by means of the two caudal horns which lie in sheaths from which 
they may be everted. The tongue is sometimes quite long, and then the 
sheaths and the enclosed horns are, of course, of corresponding length. 
If the horns are very long, some disposition must be made of them 
when retracted. This is interestingly observed in the salamander, 
Spelerpes fuscus, where the sheaths of the horns run down the sides of 
the body until they are attached to the pelvic bones, the ilia. In the 
woodpecker they pass around the occipital region over the top of the 
head and end near the anterior nares of the base of the upper beak. In 

Digestive System 


such cases the ends of the horns are fastened to the bottom of the 
sheaths so that the sheath is turned inside out when they are withdrawn. 
The tongue develops between the hyoid and mandibular arches (Fig. 
432). The hyoid often extends into and supports the tongue. Conse- 
quently, the organ itself cannot be moved unless its supporting skeleton 
is likewise moved. The tongue is a sensory organ but can also be used 
as an organ of touch and taste. There are little elevations (Fig. 433) 
known as papillae, in many if not most animal tongues. Some of these 

Fig. 432. 

Two stages in the development of the tongue and pharyngeal 
floor of man, c, copula (basihyal element); cs, cervical sinus; ep, 
epiglottis; g, glottis; h, hyoid arch; md, mandibular arch; mth, 
median anlage of thyroid gland; t, tuberculum impar; tg, tongue. 
(From Kingsley after His.) 

Fig. 433. 

Papillary surfaces of the human 
tongue showing fauces and tonsils. 
1, 1, circumvallate papillae, in front 
of 2, the foramen caecum; 3, fungi- 
form papillae; 4, filiform and con- 
ical papillae; 5, transverse and 
oblique rugae; 6, mucous glands at 
the base of the tongue and in the 
fauces; 7, tonsils; 8, part of the 
epiglottis; 9, median glosso-epi- 
glottidean fold (frenum epiglottis). 
(From Hill after Sappey.) 

are sensory while others have become hardened and serve as rasping 

In the cyclostomes the tongue is thick and fleshy and is supported 
by a cartilaginous skeleton. The muscles which throw out the tongue, 
are called protractor muscles, and those which draw the tongue back to 
its normal position, are known as retractors. These muscles are devel- 
oped from the postotic myotomes and their nerve supply comes from the 
hypo-glossal nerve. In the myxinoids the terminal end of the tongue 
possesses epidermal teeth which form a boring organ by which these 
animals obtain entrance into their prey. In the lampreys, the surface 
has a rasping organ and also forms part of the sucking apparatus. 
Among the amphibians there are a few anura (aglossa) in which the 
tongue is practically absent, but in most cases the tongue actually con- 
tains intrinsic muscles supplied by the hypo-glossal nerve when the 
tongue can be moved quite readily. The tongue of amphibians is made 

316 Comparative Anatomy 

up of a small basal portion, quite similar to that of the fish, but to this 
is added a large glandular part which develops between that portion 
called the copula, or medial region, and the lower body. The amphibian 
tongue secretes slime which is rather useful in capturing its prey. In 
anura the tongue is fastened to the margin of the jaw, while its free 
end when not in use lies on the floor of the mouth. In urodeles a much 
greater portion of the tongue is attached than in anura, for here not 
only the anterior margin of the tongue but a part of the ventral surface 
as well, is held quite definitely in place by attachments. 

The supporting skeleton of the tongue, as mentioned above, usually 
consists of two pairs of horns largely formed from the ventral ends of 
the hyoid and first branchial arches. The median portion, or body, which 
unites these horns is known as the copula. The reptilian tongue includes 
the parts already mentioned which are found in the amphibia and, in 
addition, a median growth which arises between the basi-hyal and the 
lower jaw known as the tuberculum impar (Fig. 432, t). Added to this, 
there is found a pair of lateral folds lying above the first visceral (man- 
dibular) arch. From now on, as these parts develop, the trigeminal 
nerve sends twigs to the tongue in addition to the hypoglossal and 
glossopharyngeal as in the lower groups. In turtles and crocodiles the 
tongue lies on the floor of the mouth and cannot be protruded. In 
reptiles possessing a retractile tongue the hyoid apparatus extends into 
that organ. The unpaired cephalic portion, which we have called the 
OS entoglossum, is equivalent to the term copula or basi-hyal; the 
retractor muscles are usually attached to the two horns. In the tongues 
of birds the lateral parts of the reptilian tongue are not to be found and 
consequently there is no branch from the trigeminal nerve. It is to be 
remembered that during the embryological development of a part, the 
nerves follow the growing muscle. The bird's tongue has no intrinsic 
muscles. It has many varieties of form but is usually slender and cov- 
ered with horny papillae. Even its skeleton is reduced ; there is only 
an OS entoglossum with a pair of structural elements attached in front, 
known as the paraglossae, while, on the sides, a pair of horns form the 
first branchial arch, and, in the median line behind, a portion called the 
urohyal is found. This is well marked in the woodpecker as already 

Now we shall discuss the use of the tongue. With the exception of 
the whale, the tongue is very mobile in all forms of mammalian life. 
The mobility reaches its extreme in the ant-eaters (Fig. 387). It is 
largely due to the intrinsic muscles which have been derived to a con- 
siderable extent from the hypo-branchial musculature. The tongue itself 
is developed from the unpaired elevation — the tuberculum impar — and 
from two thickenings on the mandibular arch, which, together with the 
fleshy ridges above the hyoid arch, form the tongue. These fleshy ridges 
above the hyoid arch form the back part of the tongue. The line formed 

Digestive System ?A')! 

between the anterior and posterior parts cannot readily be seen in the 
adult, but it is quite close to the circumvallate papillae and the foramen 
caecum. This latter is a little open place, or pit, in close relationship 
to the development of the thyroid gland. 

It will thus be seen that the mammalian tongue is quite sim.ilar to 
that of reptiles and exceeds that of birds by having portions in it that 
come from the mandibular arch. 

Two views are usually held as to the relations of the mammalian 
and amphibian tongues. One holds that the amphibian tongue is entirely 
unrepresented in the mammals unless it be by the sublingua. This is a 
fleshy fold beneath the tongue of marsupials and lemurs, traces of which 
occur in other mammals, even in man, as folds (plicae fimbriatae) 
beneath the tongue. In some cases (Stenops), the sublingua is sup- 
ported by cartilage, which may be the entoglossum. The other view is 
that at least the anterior part of the tongue in amniotes is quite like 
that of amphibia. This view holds that the lyssa (a vermiform mass 
of cartilage, muscle, and connective tissue, lying ventral to the median 
septum of the tongue), is the equivalent of the entoglossum and its 
associated structures. 

The dorsal surface of the tongue is covered with a soft epithelium 
with many mucous glands. There are also varying forms of papillae 
(Fig. 433), some of which, the taste buds for example, are sensory, while 
some become cornified to form epidermal teeth. A rasping type of 
tongue in which many of the papillae have become cornified is that of 
the cat. 


In animals that live under water it is quite natural that compara- 
tively few glands should be found in the mouth cavity other than the 
very simplest kind. These pour out a slight amount of mucus. If glands 
were to exist there to any extent, their secretions would be washed 
away with the incoming and outgoing water that passes through the 
mouth cavity of such animals. Then, too, one can easily understand that, 
where a secretion of an animal gland is soluble in water, if such animal 
lives in water, the secretion could be of no value whatever. Contrasted 
to this, it can also readily be understood that animals which breathe air 
must have many glands moistening all surfaces constantly, or the absorp- 
tion, which is always going on, would soon have all parts of our bodies 
so dry that they could no longer function. For this reason terrestrial 
animals have many more secreting glands than water animals. 

Mammals, therefore, have salivary glands. The saliva, which these 
secrete, contains not only mucus, but a digestive ferment known as 
ptyalin which changes starch into sugar. 

Glands are named largely after the position they occupy, such as 
labial, lingual, sub-lingual, etc. 

318 Comparative Anatomy 

In air-breathing amphibia, snakes, and lizards, there are labial 
glands opening at the basis of the teeth, an intermaxillary or internasal 
gland in the septum between the nasal cavities, as well as palatal glands 
near the choanae. These latter glands are lacking in the caecilians. 
There is a sub-lingual gland on either side in many reptiles. Probably 
all secretions from salivary glands in snakes are poisonous. There is 
only one known poisonous lizard (Heloderma). The sub-lingual glands 
furnish the poison. Birds do not have labial and internasal glands, 
but they do have numerous other glands which open separately into 
the roof of the mouth. They also have anterior and posterior sub- 
linguals and even sometimes "angle" glands at the angle of the mouth, 
a condition sometimes supposed to be a remnant of the labial glands in 
the Sauropsida. Mammals possess small labial, buccal, lingual, and 
palatine glands imbedded in the mucous membrane of the mouth. Each 
of these opens through a separate duct. All of these glands serve to 
keep the various surfaces moist. 

Many glands, however, have become specialized; for example, the 
intermaxillary glands of frogs and toads (opening into the roof of the 
mouth) secrete a viscid and sticky fluid which the tongue uses as it is 
thrown out to catch and hold insects and other moving objects. So, too, 
the buccal glands of poisonous serpents furnish the venom which is sent 
forth through the poison fangs. These poison fangs, it will be remem- 
bered, are teeth, and they are provided either with a groove along the 
external surface or else they have a very small lumen through the center 
of the tooth and act very much like a hypodermic needle. Those glands, 
which assist in throwing out a thin watery lubricant, are called serous 
glands, while those assisting in softening and dissolving dry food so 
that it can be more easily swallowed, are called salivary glands without 
regard to their position. 

In mammals the salivary glands are the parotid, lying ventral to the 
ear (swelling up in man when he has mumps), the submandibular (called 
submaxillary in human anatomy), the sublingual, and the retrolingual. 
This last one is closely associated with the submandibular. It is not 
found in all mammals. 

The serous glands secrete a clear fluid without any salivary attribute. 
The molar gland of ungulates and the voluminous orbital gland of dogs 
are examples of this type. The orbital glands open into the mouth- 
cavity close to the last upper molar. The submandibular is found in 
the lower jaw beneath the mylohyoid muscle. Its duct (Wharton's 
duct) opens near the lower incisor teeth. The retrolingual gland is near 
the submandibular with its duct opening close to Wharton's. The sub- 
lingual gland lies between the tongue and the alveolar margin of the 
lower jaw. It empties through several ducts. The parotid opens 
through Stenson's duct near the molars of the upper jaw. 

Digestive vSystem 



We have already described the pharynx as the cephalic end of the 
digestive canal lying between the cavity of the mouth and the oesopha- 
gus from which the respiratory system develops. It will be described 
in more detail in our discussion of the respiratory system. But, as 
there are certain more or less significant organs developed in the 
pharyngeal region, it may be well to discuss them at this point. These 
are especially the thymus and thyroid glands. It is customary to trace 
the development of these two glands from the cyclostomes upward 
because the cyclostomes furnish the first (more generalized) stage of 
development of such glands and thus make it possible to follow up 
consecutively any so-called advance from a lower developmental type 
to a higher one. (Figs. 294, 434.) 

There are six pharyngeal pockets (except in the cyclostomes) 
developed on each side. Each of these pockets possesses a dorsal and a 
ventral recess. It is around these recesses that a group of epithelial 
cells develops an organ-anlage quite alike in the lower forms. In the 
higher forms, however, the dorsal group soon forms the thymus, and 
the ventral forms what are called epithelial corpuscles. 

These thymus-anlagen may separate from the layer where they 
originated, or they may fuse into a single elongated organ, or they may 
become constricted in number, the anterior ones disappearing. 

In the cyclostomes there are seven anlagen. In the teleosts there 
are six pharyngeal pockets, but only four anlagen, and these are all the 
more posterior ones. 

Fig. 434. 

Thyroid and thymus glands with closely related organs. A, lizard; B, 
Hen; C, Calf. car, carotid artery; h, heart;, postbrarichial bodies; 
thym, thymus; thym\ point of thymus attachment; thyr, thyroid gland; tr, 
trachea; v. jug., jugular vein. (After DeMeuron.) 

In the mammals it is the third pocket which produces the thymus- 
anlage, although sometimes there is a tiny addition from the fourth. 

The epithelial corpuscles tend to disappear, but in amphibians they 
become glandular and associate with the carotid artery to form carotid 

320 Comparative AnatomV 

glands. The number of these carotid g^lands varies in different groups 
of animals. 

Immediately behind the last gill slit, in the floor of the pharynx, 
there is a pair of evaginations. These have been termed supraperi- 
cardial bodies because they secondarily become associated with the 
pericardium of the selachians. At present they are usually called post- 
branchial bodies. In selachians a complete pair of these bodies develops 
but in urodeles and lizards only the left one ever completes its develop- 
ment, the right ultimately disappearing. Whether these bodies occur in 
birds and mammals is not known, although there are somewhat similar 
growths, called parathyroid bodies, which do develop in these animals 
and then become lost in the lobes of the thyroid gland. 

Explanations of these bodies are not yet satisfactory. 

The thyroid has come to be considered a very important organ since 
endocrinology looms up so large in the medical world. This gland is 
an evagination of the pharynx. It is first seen in the selachians but 
makes its appearance regularly in the higher forms. It arises from the 
floor of the pharynx at about the level of the interval between the first 
and second pockets. It becomes compact, and, like the thymus, does 
not develop a duct. In the larva of Petromyzon (one of the cyclostomes) 
the thyroid appears as an open trough, lined with cilia, which is in open 
communication with the pharynx, a position quite like that in Amphi- 
oxus. This trough is called the hypo-branchial groove, or endostyle, an 
organ which assists the passage of food down the pharynx by exuding 
a slimy secretion and by furnishing a definite track, with cilia, which 
can thus facilitate its movement. 

Professor Wilder thinks the thyroid gland is primarily a digestive 
organ, although in the true vertebrates its structure, as well as its 
function, has nothing to do with digestion. It is now generally thought 
that the internal secretions of the thymus gland stimulate growth and 
inhibit development while the secretions of the thyroid gland stimulate 
development and inhibit growth. 


This is the swallowing tube connecting the mouth with the stomach. 
It lies directly against the interior of the dorsal wall of the body-cavity 
and thus lacks a serous coat. There are no digestive glands in its walls 
as a rule. Its length quite naturally varies with the length of the neck 
of the animal in which it occurs. Usually its internal lining is a smooth 
epithelium. In the chelonians one finds cornified papillae pointing back- 
ward. The oesophagus, like other parts of the digestive tract, consists 
of five layers ; however, as it will be remembered that the ectoderm has 
indented to form the anterior and posterior openings into the digestive 
tract, a histological examination in the region where these two divisions 
merge into each other will show a change of structure. 

Digestive System 321 

The muscles contained in the walls are striated at the cephalic end 
and extend back in some cases even into the stomach. The oesophagus 
usually has the same diameter throughout, but in many, if not most 
birds, there is a dilation called the crop or ingluvies. This may either 
be an expansion on one side, or, as in pigeons, it may consist of a median 
as well as a pair of lateral chambers. The crop may be a simple reser- 
voir for food, or it may be a real glandular organ where secretions are 
poured forth and digestion started. In fact, during the breeding season 
pigeons secrete a milky fluid here, which is used in feeding the young. 


The various portions of the stomach have already been named and 
described in the frog. To the terms there given should be added the 
small curvature at the top or anterior surface of the stomach, usually 
called the "lesser" curvature, and the posterior curvature called the 

In some forms of animals, such as amphibians, the lining of the 
mouth, oesophagus, and stomach is covered with cilia. In birds the 
stomach is divided into an anterior glandular region, called the pro- 
ventriculus, and a posterior muscular region, called the gizzard. After 
the food has passed through the proventricular region and has mixed 
with the secretion from its glands, it passes into the gizzard. This 
latter organ is not only muscular, but the muscles have developed into 
a pair of disks with tendinous centers. There is a secretion in the 
gizzard which hardens the lining, and, sometimes, even raises little 
elevations which are used in grinding the food. One might almost con- 
sider them teeth. Remembering that birds have no true teeth, one can 
readily understand the advantage such an animal has in a gizzard of 
this type. Grain-eating birds swallow small pebbles which enter the 
gizzard and are thus also made use of for grinding purposes. 

In fact, in the fossil pterodactyl pebbles have been found in such 
portions as to lead to the supposition that these reptiles had a gizzard. 
It is well to note in this regard that the grain-eating birds have the 
best developed gizzards, while birds of prey have gizzards much less 
fully developed. In one species of pigeons, a part of the wall of the 
gizzard is ossified. In mammals there are more varying forms of stom- 
achs. These are divided ,in from one to four regions. The ruminants 
have two well developed divisions of the stomach (Fig. 435), the rumen 
or paunch, and the reticulum or honey comb, though these two divisions 
are really enlarged portions of the oesophagus and serve as reservoirs 
of food. The food is regurgitated into the mouth for mastication and, 
after it is swallowed a second time, passes into the true stomach, the 
psalterium (also called omasus or manyplies), and then to the abomasum 
or rennet. The latter is used for gastric digestion. 

It is of interest here to trace the embryonic changes of the mesentery 


Comparative Anatomy 

in mammals. The mesentery supporting the stomach is called the meso- 
gastrium. The first curvature of the stomach, v^hich is toward the left, 
broadens the corresponding part of the mesogastrium, an effect which is 
still further increased by the lateral torsion of the entire stomach. The 
spleen develops within this widened part and by its weight produces 
a fullness which in turn causes a sagging down behind (dorsal to) the 
lesser curvature, although attached to the greater. This tendency con- 
tinues and causes the free lower fold of the bag-like extension to hang 
down behind the stomach (Fig. 427). 

This fold is called the greater omentum (omentum majus), which, 
as all mesenteries are essentially double, must consist of four layers of 
serous membranes, applied two and two, each pair holding between them 
the blood vessels and absorbent vessels naturally belonging to a mesen- 

Fig. 435. 

Stomach of a Sheep. 4 abom, abomasum; d, intestine; f^f^, 
two folds which divide the rumen (paunch) into three regions; 
kl, pyloric valve; 6 (3-4) opening which leads from the third 
to the fourth stomach region; oes, oesophagus; 3, psalt, psalterium 
(omasus or manyplies); 2.ret, reticulum (honeycomb); l.rum, 
rumen (paunch) ; schl.r., pharyngeal groove. 

The piece of wire marked / shows the direction the unmasti- 
cated food takes, while II shows the direction of the remasticated 
food. (After Carus and Otto.) 

tery. The cavity of the bag is the lesser peritoneal cavity of human 
anatomy, and the opening into it (behind the stomach) is the foramen 
epiploicum (foramen of Winslow). The bag is widely open in most 
mammals but in man the foramen is considerably reduced in size and 
the layers forming the pendulous fold are fused together to form a 
four-layered apron that hangs below the stomach and covers the intes- 
tinal folds. 

The duodenum takes its name from the twelve inches of rather 
large diameter intestine which immediately follows the stomach. The 

Digestive System 


word ''duodenum" is a name which was taken from human anatomy 
though even in man the structure bearing that name is closer to eleven, 
inches in length than it is to twelve inches in the adult. In the lower 
forms of animals it varies in length and shape as do all the other parts 
of the intestinal tract. 

Growing from this portion of the intestinal tract, immediately 
beyond the pyloris, in some of the ganoids and teleosts there may be as 
many as one to two hundred blind tubes. These are known as the 
pyloric caeca. There are a few elasmobranchs which have only one 
pair of these caeca. The caeca may be expanded into a pouch called a 
bursa Entiana. The region of the intestine running caudad from the 
duodenum is also called the post-hepatic intestine, so it is in this region 
caudad to the liver, where most of the digestive processes as well as 
most of the absorption of the products of digestion take place. 

The food, having been more or less mixed with various salivary 
secretions and having been reduced to a semi-liquid state, receives the 
bile from the liver and the pancreatic juice from the pancreas (Fig. 436). 

Fig. 436. 

A, The duodenum of a rabbit with vine-like pancreas. P., Pyloric end of 
stomach; gh., gall bladder with bile duct and hepatic ducts; p.d., pancreatic duct. 
(From Krause after Claude Bernard.) 

B, Appendix vermiformis of kangaroo; C, Appendix vermiformis of human 
embryo. (After Wiedersheim.) 

It is then ready, after being moved back and forth by the peristaltic 
movement of the intestine, to be taken up and absorbed by the little 
finger-like processes (villi) which extend from the inner surface of the 
small intestine. Here it is well to remember that the length of the 
intestine varies with the type of food the individual eats. It is longer 
in plant-eating animals than in meat-eating animals. A classic illustra- 
tion of this is the comparative length of the intestine in the adult frog 
and in the tadpole. The adult frog's intestine is no longer than 
that of a tadpole half the size a frog would be if it had kept up its 
relative increase in size. 

Any blind pouch in an animal is called a caecum. At the beginning 
of the large intestine where the small intestine enters into it, the joining 
itself is called the ileo-caecal junction, and the little projecting end of 

324 Comparative Anatomy 

the large intestine is the caecum. It is the tapering end of this caecum 
which forms the appendix (Fig. 436) in man and in some of the other 
vertebrates. There is also a valve at the junction of the ileum and 
caecum, known as the ileo-caecal valve. 

The first part of the small intestine, which follows the duodenum, 
is known as the jejunum, while the more distal portion is the ileum. 

Professor Wilder gives the following interesting account of the 
appendix and succeeding structures: 

"At the junction of the small intestine with the large, there is a 
strong tendency to form one or more caeca, or blind sacs, which often 
become digestive organs of great physiological efficiency. The charac- 
teristic form in reptiles is that of a single rather short and wide caecum, 
symmetrically placed. In birds there are usually two symmetrical ones, 
which attain great length in scratching birds (e. g., the common fowl), 
and in ducks and geese, but are quite rudimentary in certain others 
(woodpeckers, parrots, etc.). Ostriches possess a single caecum of 
great length (seven to eight meters) and furnished with an internal 
spiral partition, which greatly increases its effective surface. 

*Tn mammals a single caecum is developed, which varies greatly 
in size and functional importance. Rudimentary in edentates, most 
insectivores, and bats, it frequently attains an enormous size in herbiv- 
orous or graminivorous forms. In certain rodents (e. g., muskrat, 
woodchuck), its total capacity equals or exceeds that of the remainder 
of the alimentary canal, and in the marsupial Phascolarctus it is three 
times the length of the body. In the rabbit it is provided with an 
internal spiral valve; in certain other rodents and in the higher apes 
and man, the free end becomes rudimentary, restricts its lumen, and 
forms a worm-like process, the processus (appendix) vermiformis, which 
like all rudimentary organs, is subject to a large amount of individual 

"Thus in the human subject the appendix varies in length between 
the limits of 2-23 cm., the average for an adult being 8-9 cm. It is 
longest proportionally during fetal life, its length relative to that of 
the large intestine being 1 :10, while in adult life it is 1 :20. It is longest 
absolutely between the ages of ten and twenty, after which it shows a 
slight reduction. Its status as a rudiment of slight functional value is 
shown by the tendency toward the obliteration of its lumen, a tendency 
which increases steadily with age. Furthermore, these two characters, 
reduction in length and obliteration of the lumen, go hand in hand, 
short appendices being usually solid, while large ones are likely to 
possess a lumen. 

"The position and arrangement of the colon varies considerably 
among various mammals. In man it begins low down on the right side, 
from which there proceed in order an ascending, transverse, and descend- 
ing portion, connected with the rectum by a sigmoid flexure, through 

Digestive System 325 

which the tube attains the median line; a similar disposal is seen in 
many other anthropoids, in lemurs and rodents, the majority of carni- 
vores, and a few others. A more complex condition than this is produced 
by the formation of long, narrow loops along the course of either the 
ascending or transverse colons, or both, and these loops may remain 
simple or roll into spirals. Such colon labyrinths are seen in ruminants, 
in certain rodents as the lemmings and jumping mice, and in a few 

"From this brief review of the alimentary canal and its modifica- 
tions the impression is gained that in this array of enlargements, elonga- 
tions, diverticula, spiral valves, and other devices, we have to do, not 
with a consecutive anatomical history, but with numerous special cases 
of physiological adaptations, developed in response to need ; and that a 
similarity in one of these particulars implies, not genetic relationship 
necessarily, but a similar demand responded to in a similar way. The 
main object to be achieved in all cases is to regulate the amount of 
digestive surface to the demands ofiFered by the various kinds of food, 
and as there is but a limited number of mechanical or architectural 
devices possible, the same ones are employed in unrelated groups of 
animals, having arisen independently in response to a similar physio- 
logical need. This phenomenon of parallel development (or 'analogical 
resemblance,' as Darwin calls it), may appear in any system or part 
and has been a frequent source of error in the estimation of the inter- 
relationship of animals." 

It has already been shown that the intestinal tract begins as a 
straight tube ; enlargements then take place in various portions of this 
tube, the most prominent of such enlargements being the stomach. 
This enlargement has various paired nerves passing down each side of 
the digestive canal, prominent among which are the vagus nerves. When 
the stomach has become sufficiently large and extends some distance 
ventral, it turns, so that what was the ventral region now points toward 
the right side of the individual. This means that any nerves or blood 
vessels which lie along the right side of the embryonic digestive tract 
will then lie on the dorsal surface of the stomach and, of course, the 
left nerves and blood vessels then become ventral. It will save con- 
siderable confusion of thought if this be remembered. 


The liver, as well as the pancreas (Fig. 293) — the two largest diges- 
tive glands — are derived from the mucosa of the intestine. The former 
grows by a ventral, and the latter by a dorsal, evagination. These 
organs are in a sense enlarged intestinal glands which pushed their way 
through mucosa, submucosa, and muscularia of the intestine. Then, 
as they pushed against the serosa (which is held down very loosely), 
this tissue stretched and grew directly ahead of the two glands. The 

326 Comparative Anatomy 

liver and pancreas are, therefore, covered by a serous membrane, and 
both are connected on the side from which they pushed forth by a double 
layer of serosa which forms respective mesenteries. This serous cover- 
ing is continuous with the covering of the entire intestinal tract and 
is known as the visceral peritoneum. The peritoneum also forms the 
lining of the abdominal cavity. 

The liver is the largest gland in the body but, no matter how many 
lobes it may develop or how large it may grow, a layer of serosa covers 
every part of the gland except that part lying toward the side from 
which it grew. Here, as stated above, the layers coming from each side 
naturally unite and form the double layer of serosa — the mesentery. 
The two large suspensory mesenteries of the liver are called the ligamen- 
tum hepato-gastricum (sometimes also called the lesser omentum) and 
the ligamentum suspensorium-hepatis. 

Practically the entire length of the digestive canal, which passes 
through the body-cavity, was originally attached by both a dorsal and 
ventral mesentery. The ventral mesentery, however, becomes lost below 
the region of the liver, leaving a sharp ventral edge to the two hepatic 

The function of the liver is to secrete bile (gall), as well as to form 
various internal products such as glycogen, urea, and uric acid, all of 
which substances are of great importance to the living animal. The 
bile is sent to the intestines through the bile duct (also called the 
choledochal duct) while the other products are carried away by the 
blood. Substances secreted by glands which are not sent through a duct, 
but carried throughout the body by the blood stream, are known as 
substances of internal secretion. 

The liver is a compound tubular gland. The many little tubules in 
the liver which form the gall capillaries empty into the bile duct. This 
tubular condition of the liver is easily seen in ichthyopsida but is difficult 
to observe in mammals because of the tubular anastomosis and because 
of the close interrelation of the bile vessels and blood vessels. 

The liver begins its growth cephalad at about the same time the 
blood vessels have already developed into the large sinus venosus and 
hepatic veins. These blood vessels also contribute to the septum trans- 
versum (Fig. 348). The growth of these latter organs prevents the liver 
from continuing its cephalad growth so that from now on it increases 
in size in an opposite direction. 

Concomitant with its increase in size there is an immigration of 
mesenchyme between the lobules of the liver. The blood vessels enter 
at this time. The bile duct (if there are several, this is only true of one 
of them) has a lateral diverticulum or enlargement. This is the gall 
bladder (Figs. 426, II, 436), which serves as a reservoir for the bile. It 
may be found in the substance of the liver itself but is usually more or 
less separate and lies dorsal to the liver substance. It is lacking in some 

Digestive System 327 

mammals. In fact, it is not uncommon in man to have the gall bladder 
removed surgically. 

Both the liver and the gall bladder have ducts leading from them. 
Those coming from the liver are called hepatic ducts; those from the 
gall bladder are called cystic ducts (Fig. 436). These unite to form 
the common duct which is also called the choledochal duct. It is this 
common, or choledochal, duct which empties into the intestine. The 
liver has many and varying shapes in the different animals, depending 
to a large extent not only upon the shape of the body but on the shape 
and size of the organs which press upon it. The color of the gall may 
vary from a brown, yellow, purple, or green, to a vermilion. 


This is the second largest of the digestive glands (Fig. 436) and 
secretes digestive ferments of great strength, such as trypsin, steapsin, 
and amylopsin, which digest both proteins and carbohydrates. 

In some respects the pancreas resembles the salivary glands and so 
compensates in part for the absence of such glands in the lower verte- 
brates. This pancreas arises, as already mentioned, from the dorsal wall 
of the intestine, close to the liver. There are usually three diverticula, 
one dorsal and two ventral. These latter soon unite. In sharks there 
is only a single diverticulum, while in the sturgeon there are not only 
two dorsal but also an equal number of ventral. The proximal portion 
forms the ducts, the distal, the glands. The number of ducts that per- 
sist varies immensely. In some forms of animals all but one disap- 
pear, while in the lampreys all may be lost. However, in the mammals 
two ducts usually persist : the ventral, known as the pancreatic, or 
Wirsung's, duct, and the dorsal called the accessory, or Santorini's, duct. 
Again, the ducts may all remain distinct, or they may unite before they 
enter the intestines. One of them may even unite with the bile duct. 
While not absolutely proved, it seems that all vertebrates have some 
form of pancreas. This may be only a slender tube in the mesentery, 
as in teleosts, or it may lie outside the muscles in the intestinal walls, 
as in dipnoi. In the cyclostomes, it is partly concealed at the insertion 
of the spiral valve and partly in the liver. In these forms, however, 
the duct has entirely disappeared so that it forms one of the ductless 
glands or, in other words, a gland of internal secretion. The pancreas 
varies in shape and size. It may be long and straight or possess many 
lobules. Almost always it is placed between the duodenum and the 
stomach. There is a question as to whether or not the gland is composed 
of two separate and distinct structures. 


The mouth lies at the bottom of a vestibule (Fig. 437) as an oral 
funnel bounded by ciliated buccal tentacles with cartilaginous supports 


Comparative Anatomy 

Fig. 437. 

Amphioxus lanceolatus: a, 
Anus; au, eye; b, ventral 
muscles; c, body cavity; ch, 
notochord; d, intestine; do 
and du, dorsal and ventral 
walls of intestine; /, fin-rays; 
h, skin; k, gills; ka, gill- 
artery; lb, liver; Iv, liver- 
vein; m^, brain vesicle; m^, 
spinal marrow; mg, stomach; 
o, mouth; p, ventral pore; r, 
dorsal muscle; s, tail fin; t, t, 
aorta; v, intestinal vein; x, 
boundary between gill intes- 
tine and stomach intestine; y, 
hypobranchial groove. (After 

that serve to funnel the v^ater into the pharynx. 
The mouth is surrounded by a membrane, the 
velum, which acts as a sphincter muscle. A set 
of velar tentacles that serve as a grating to strain 
out the larger particles is developed on the free 
edges of the velum. 

The pharynx has sometimes upward of fifty 
or more pairs of gill-clefts (also called branchial 
apertures) that are separated by partitions in 
which lie cartilaginous skeletal rods, connected 
across with one another, forming a sort of 
branchial basket. These apertures serve as 
means of communication between the pharynx 
and the atrium (the space between the pharynx 
and the body-wall). The endostyle (a longitudi- 
nal groove on the ventral side of the pharynx) 
as well as the peripharyngeal and hyperpharyn- 
geal grooves all secrete mucus in the form of a 
continuous rope which carries the food along 
with it to the stomach. The atrium is a sort of 
mantle composed of folds of the body-wall that 
enclose the whole branchial apparatus in a 
voluminous water-filled chamber, the atrial 
cavity. The atrium is lined with ectoderm and 
has but one opening to the exterior, a posteriorly 
directed atriopore, which carries off the water 
that comes through the pharyngeal clefts. The 
atrium is a protection for the delicate pharynx, 
while the animal is in its sandy burrow, and helps 
to maintain an uninterrupted current of water. 

In the Ascidians (Fig. 313, IV) the method 
of food concentration and transportation is simi- 
lar to that of Amphioxus although the apparatus, 
which carries on this function, seems to be of an 
improved type more appropriate for a sedentary 
life. An atrial cavity surrounds the pharynx 
which in turn is enclosed by a mantle that sur- 
rounds the whole body. A thick tunic (after 
which the animal takes its name) covers this 
mantle. The atriopore is not posterior in direc- 
tion but lies close to the mouth and is forwardly 
directed. The stomach opens near the bottom of 
the pharynx, and the intestine takes a complete 
turn and opens forward into the atrium. There is 

Digestive vSystem 


no notochord and no neural tube. Practically none of the structures 
characteristic of the dorsal side of Amphioxus are present. 


The mouth opens directly into the capacious pharynx, which is per- 
forated by five gill-slits and the paired spiracles. A short oesophagus 
of large caliber leads into a U-shaped stomach (Fig. 438), v^hich in turn 

Fig. 438. 

A female dogfish in which the abdominal and pericardial cavities have been 
opened from the ventral side, and the viscera somewhat displaced. The pericardium 
has been opened slightly to the left of the middle line, and the right lobe of the 
liver has been cut away. 

ab.p., Abdominal pores; b., bile duct; c, cardiac limb of stomach; car., caudal 

330 Comparative Anatomy 

communicates with the intestine through a valve-shaped opening con- 
trolled by a sphincter muscle. The cardiac end of the stomach may end 
as a blind pouch. The organ is often sufficiently distensile to permit one 
animal to sw^allow another as large as itself. The intestine is short but 
of large diameter and has a secreting surface greatly enlarged by a fold 
in the shape of a spiral staircase (present, how^ever, in very few teleostei) 
called the spiral valve. All primitive fish have this spiral valve. A large 
bi-lobed liver, which is provided with a gall bladder and a bile duct, 
opens into the intestine. The pancreas also pours its secretion into the 


The digestive system of reptiles varies somewhat in carnivorous 
and herbivorous forms but in all turtles it is comparatively simple. 
There are no teeth. The tongue is broad and soft and cannot be pro- 
truded. The stomach is a simple U-shaped enlargement of the alimen- 
tary tract. The intestine is without a caecum ; it is clearly divided into 
large and small intestines. The cloaca is proportionately large. 


The mouth is hard and narrow and the tongue is hard and often of 
great functional value. The oesophagus, which has many large cornified 
papillae, develops an enlargement called the crop. The stomach has 
a proventriculus, which secretes the gastric juice, and a muscular gizzard 
or gastric mill. The intestine is U-shaped, and is composed of duo- 
denum, ileum, and rectum. Between the ileum and rectum there are 
two caeca. The rectum opens into a cloaca. There are two bile ducts 
but no gall bladder. The pancreas empties into the duodenum. The 
intestine is longer and more coiled than in lower forms. In cyclostomes, 
teleostomes, and all non-placental mammals, the intestine terminates in 
a cloaca, as do also the urinary and genital ducts. In placental mam- 
mals, in cyclostomes, and in teleostomes, the urinary and genital ducts 
have a distinct and separate opening from that of the intestine. 


In all vertebrates (except birds and mammals) the coelom consists 
of the following two compartments : 

artery; c.v., caudal vein; d, bursa Entiana; /./., falciform ligament appearing on 
surface of left lobe of liver in which it is embedded; i., intestine; i.a., intestinal 
branch of anterior mesenteric artery; /, lienogastric artery; not., notochord; ov., 
ovary; p., portal vein lying beside hepatic artery; ps., pancreas with duct opening 
into intestine; py., pyloric limb of stomach; r., rectum, between hinder ends of 
oviducts, with rectal gland ( attached to its dorsal side; sh., right shell gland 
on course of right oviduct; sp., spleen; .y^.c, spinal cord; Mr./>., urinary papilla; 
v., branch of portal vein formed by junction of intestinal and splenic veins. 

Besides the above, note — nostrils; oronasal grooves; mouth; pectoral and pelvic 
fins; pericardial and abdominal cavities; heart, consisting of sinus venosus (behind), 
ventricle, auricle (showing at sides of ventricle), and conus; cloaca, and transverse 
section of tail, showing at the sides the myomeres, above the anterior dorsal fin, 
and in the middle the cartilage of the backbone enclosing spinal cord, notochord, 
and blood vessels. (After Borradaile.) 

Digestive System 


(1) The pericardial cavity which contains the heart only. 

(2) The pleuroperitoneal cavity which contains the other viscera. 



"if ■•Ipfc ^11 


-— ' ■ f iw w^? ^ 

T----^B^ III// I 



Fig. 439. 

A dissection of the neck and thorax of a rabbit. The heart has 
been displaced a little to the right, and the pericardium removed. 

ao.a.. Aortic arch; c.c, common carotid arteries;, cervical 
sympathetic nerve;, dorsal aorta; dep., depressor nerve; di., 
diaphragm;, ductus arteriosus; ex.f., external jugular vein; f.c, 
point at which the common carotid divides; hy., hypoglossal nerve; 
i.c.g., inferior or posterior cervical sympathetic ganglion; inn., innomi- 
nate artery; i.v.c, inferior vena cava, lying in mediastinum;, 
left auricle; /./., left lung; l.phr., \&i\. phrenic nerve;, left pleural 
cavity; l.v., left ventricle; lar., larynx; ces., oesophagus in neck; (es\, 
the same in mediastinum; p.c, posterior cornu of the hyoid; pul.a., 
pulmonary artery; pul.v., pulmonary vein;, right auricle; r.d., 
ramus descendeus; r.l., right lung, one part bulging into mediastinum; 
r.lar., recurrent laryngeal nerve;, right pleural cavity; r.v., right 
ventricle; s.c.g., superior cervical sympathetic ganglion; s.lar., superior 
laryngeal branch of vagus; s.v.c, superior vena cava; scl., subclavian 
artery and vein; smx., submaxillary gland; t.m., tendon of mandibular 
muscle; thy., thyroid gland; tra., trachea; v.g., vagus ganglion; vag., 
vagus; W.d., duct of submaxillary gland (Wharton's duct); X., XII., 
cranial nerves. (From Borradaile.) 

332 Comparative Anatomy 

A partition, the transverse septum (Fig. 348), separates the two 
cavities. In vertebrates lower than Anura, the pericardial cavity lies 
cephalad to the pleuroperitoneal cavity. Beginning with the Anura, 
the pericardial cavity comes to lie ventral to even the cephalic end of 
the pleuroperitoneal cavity, because the heart and the pericardial cavity 
descend and carry the transverse septum with them. This descent 
causes the wall of the pericardial cavity, together with the transverse 
septum, to form a sac — the pericardial sac — around the heart. 

The part of the pleurocardial cavity dorsal to the heart later becomes 
the pleural cavities. 

The pleuroperitoneal cavity in birds and mammals divides into 
anterior and posterior regions by a partition which descends from the 
dorsal body-wall to unite with the transverse septum. This partition 
is known as the oblique septum in birds and the diaphragm in mammals 
(Fig. 439). In mammals this diaphragm contains a great amount of 
striated muscle. 

The coelom in birds and mammals has become divided into four 
compartments : one pericardial, two pleural, and one peritoneal cavity. 

While a dorsal mesentery supports the digestive tract in all verte- 
brates, the ventral mesentery is absent in the adult except in the regions 
of the liver and bladder. 

In mammals, the mesentery of the stomach is prolonged posteriorly 
to become the greater omentum. An ileo-colic valve and a single caecum 
are usually found where small and large intestine meet in mammals, 
although there are a few instances where there are two caeca. In some 
edentates, in bats, in some carnivorous animals, and in many whales, 
neither valve nor caecum are found. 

The caecum in some rodents and marsupials grows as long or longer 
than the animal's body (it is of great value in digestion here) while in 
man, it degenerates into the vermiform appendix, the lumen of which 
tends to close with increasing age. 

The intestine and colon in mammals are straight tubes at first but 
grow into folds later. 

In monotremes the rectum terminates in a cloaca, as it does in the 
Sauropsida. This condition also occurs in the young of all mammals, 
but, in all of these, the urogenital and digestive openings become sepa- 
rated later, and a perineal fold develops between the openings. 



IN the description of the frog it was stated that the trachea and 
oesophagus have their beginning close together at the caudal end of 
the pharynx which is also the beginning of the cephalic end of the 
larynx. In the higher forms of animals, the trachea divides into two 
bronchii. These bronchii again continually subdivide until there are 
many tiny tubules, called bronchioles, spreading out to all parts of the 
lungs. These bronchioles form a sort of an air-capillary system through 
which the inspired air is sent to all parts of the lungs, there to assist in 
aerating the entire pulmonary blood which has been sent to the lungs, 
from the heart, through the pulmonary artery. In order that the oxygen 
in the inspired air can come in direct contact with the blood itself, there 
must be a rather thin, more or less porous, membrane separating the 
blood and air. ^ 

The lungs, liver, spleen, and kidneys are known as parenchymatous 
organs. It is well to bear this in mind constantly for many diseases 
find their way from one of these organs to another. A parenchymatous 
organ is more or less sponge-like and consists of loosely woven tissue 
in which there are many porous openings. Such organs are invariably 
supplied with great quanties of blood. 

These organs, especially the lungs, have a decidedly thin membrane 
surrounding the sac-like ends of the bronchioles. In the lungs the 
oxygen passes through the thin walls to come into direct contact with the 
venous blood which has been sent there through the pulmonary artery. 
What has been said so far regarding the respiratory system applies 
to vertebrates at large. However, those which live a part of their lives 
in water have no lungs during that period and in this respect resemble 
fish and other animals which spend all of their time in the water. In 
such forms gills (also called branchiae) develop on the walls of some of 
the visceral clefts (these are called gill clefts (Fig. 295) or branchial 
clefts). The clefts come from the sides of the pharynx and begin as a 
pair of pouches or grooves of the pharyngeal entoderm. As they then 
extend toward the sides of the animal they push aside the mesoderm and 
finally reach the ectoderm. The ectoderm and entoderm then fuse to 
form a plate. This plate becomes perforated and thus connects the 
pharynx with the exterior of the body by a number of openings. These 
openings, or clefts, begin development at the cephalic end and succes- 
sively continue caudad. 

The visceral pouches, although developing in all verteberates, do 
not as a rule break through in the mammals. In fact, the pouches may 


Comparative Anatomy 

disappear without leaving any trace whatever, except a Eustachian tube 
and the various ductless glands already mentioned. In the true verte- 
brates fourteen pairs of these clefts is the largest number found. There 
are more than this in Amphioxus and Balanoglossus. In the cyclostomes 
there are usually seven (eight to seven in notidanid sharks, five or six 
in teleostomes, and five in birds and mammals). In this numbering, the 
oral cleft is not included, though there is some evidence that the mouth 
arose by the coalescence of a pair of gill clefts. 

The gill clefts do not form a serial repetition in the same manner 
as does segmentation in other parts of the body, and it may even be that 
the metamerism of the head is not of the same character as the 
metamerism of the gill clefts. In the amniotes where gills are never 
developed, the branchial pouches, or clefts, however, appear and bear 
practically the same relation to the aortic and branchial arches as in the 
lower forms. From this it is often assumed that all of these higher forms 
which show this relation, have had ancestors with gills. 

There is an interbranchial septum covered externally with ectoderm 
and internally with entoderm between every two successive gill clefts. 
The inner portion of this septum is composed of mesoderm which in its 
earlier stages contains a diverticulum of the coelom. Later, blood-ves- 
sels (aortic arches) and skeletal elements (visceral arches) are developed 
in each septum. The visceral arches form on the splanchnic side of the 
coelom and hence are not comparable to girdles or ribs. 

In Cyclostomes and fishes, the gills are either filamentous or lamellar 
outgrowths of epithelium, which have developed on both anterior and 
posterior walls of the interbranchial septa. Each gill contains a loop 
of blood-vessel. There are two very thin layers between the blood and 
the surrounding water, which thus permit an exchange of gases. 

The filaments (sometimes called 
gill-plates) (Fig. 440) which bound 
each gill anteriorly and posteriorly, 
on one side, form a demibranch, and 
it is the two demibranchs of a sep- 
tum which then constitute a gill. 
This means that each cleft is 
bounded by demibranchs belonging 
to two gills. 

Some forms have external gill- 
filaments in the very young which 
are later absorbed. 

In sharks that have more than 
five gill clefts, as well as in the 
'^' ■ Cyclostomes, the first cleft bears 

Diagram of a gill. a, gill-arteries; br, 'ii v . • i v i 

branchial ray; d, demibranchs; kb, cross sec- glUs, but m many clasmobrauchs, aS 

^Z^' ('raftl Cuv"So '"'^ '' "''""^^ '' well as in the ganoids (sturgeon and 

Respiratory System 335 

Polypterus), this cleft becomes smaller and smaller until there is only 
a dorsal opening on the head — the spiracle. In most vertebrates this 
spiracle is closed in the adult, but in the tailless amphibia and the higher 
mammals the inner portion persists as the Eustachian tube and the 
greater part of the middle ear. 

There are two types of gills in fishes. Practically all the elasmo- 
branchs, with the exception of the chimaeroids, have the interbranchial 
septum well developed so that it extends beyond the demibranchs and 
thus differentiates an excurrent canal in the cleft. The prolonged sep- 
tum bends caudally at the outer end to protect the gills from injury. 

In teleostomes and chimaeroids the broad fold of the posterior end 
of the hyoid arch grows backwards over the clefts to form a gill-cover 
or opercular apparatus. The gill-cover encloses an extrabranchial or 
atrial chamber into which the clefts empty. The chamber opens by a 
single slit behind the operculum. 

In those instances, just mentioned, where an operculum is developed, 
the interbranchial septum is always reduced in size until there is only a 
slender bar from which the demibranchs extend into the atrial cham- 
ber. The two opercular folds are usually continuous beneath the 

In teleosts and ganoids the operculum (gill-cover proper) is usually 
differentiated from a more ventral portion, known as the branchiostegal 
membrane, which is quite flexible and possesses a skeleton of slender 
branchiostegal rays. The ventral wall of the pharynx in these cases is 
nothing but a slender bar and is called the isthmus. 

Just as the air in the lungs in the higher forms of animals is taken 
in through the outer air passages and then passes through the trachea, 
bronchia, and bronchioles to the delicate septa in the lungs, so in animals 
possessing gills there is likewise a delicate septum which separates the 
blood from the stream of water which is constantly being passed over 
the gills. Water is as a rule drawn into the mouth and then, as the 
enlarged oral cavity contracts, it is forced out through the clefts, passing 
over the gills on its way. In the Myxinoids the oesophageo-cutaneous 
duct probably acts as the incurrent passage when the animal has the 
front of the head immersed in the flesh of a fish. In the lampreys the 
water is probably taken and forced out through the gill clefts when the 
animal is attached to some object. The spiracle serves as an incurrent 
opening in many elasmobranchs and is provided with a valve which 
develops from the anterior wall. It closes to prevent any backflow. 
Sturgeons and Polypterus have spiracles throughout life. 

Sharks have the gill clefts on each side in the so-called neck region, 
while skates have them on the lower surface of the body. This differ- 
ence is brought about by the union of the anterior appendages with the 
head in skates. 

Many teleosts have breathing valves at the mouth-opening which 

336 Comparative Anatomy 

permit water to enter but not flow out again. In such cases there is 
a more posterior pair formed by the branchiostegal membrane closing 
the opercular opening through which the outflow of water may occur. 

In some of the teleosts and in such forms as Polypterus there is an 
opercular gill with respiratory functions developed on the inner surface 
of the operculum while in some of the elasmobranchs (even those in 
which the spiracle is closed) pseudobranchs, composed of vertical folds, 
are developed on the anterior wall of the cleft. These are homologous 
with gills, but they are not respiratory as they receive only arterial blood 
which passes from the pseudobranch to the choroid coat of the eye and 
sometimes even to the brain. 


In the amphibia, although the gill pouches form just as they do in 
fishes, the first and fifth never break through, while in nearly all adult 
forms all the clefts are closed. Exceptions occur in perennibranchs and 
the derotremes in which from one to three external openings persist. 

In the tailed amphibia and in the caecilians, the operculum is merely 
a fold of integument in front of the gill-area (Fig. 341). The operculum 
develops without a skeleton support in the larva of tailless amphibians. 
This fold grows backward over the gills and fuses. Thus atrial 
chambers are formed which usually open by a single excurrent pore 
to the exterior. In a few forms, however, both right and left excurrent 
openings occur. 

It is usually conceded that the gills of amphibia are of ectodermal 
origin and that both external and internal gills may be present at the 
same time. In the tailless amphibia, such as frogs, the operculum grows 
over the gill clefts, and the external gills are folded into the atrial 
chamber, where they are gradually reduced, while the gills which 
developed from the walls of the clefts become functional. At the time 
of metamorphosis, the clefts are entirely closed and the gills absorbed. 

It has usually been taught that the gills of fishes are entodermal in 
origin, but if this is true, they cannot be homologues of the amphibian 
gills. However, the structures are so much alike in appearance, in struc- 
ture, and in function, that it seems they must be homologous. Neverthe- 
less, more evidence must be awaited before positive assertions of value 
can be made. 

It may be interesting, and with further knowledge some time it may 
prove of value to note from the foregoing that amniotes have visceral 
pouches in the embryo, though gills are never developed in the adult; 
that reptiles have five of these pouches — birds and mammals four. In 
man only the first breaks through to form a cleft, while in many of the 
higher forms there are grooves on the outside of the neck which show 
their original position. The manner of obliterating these external 
grooves is as follows : The arches most cephalad, especially the hyoid, 

Respiratory System 837 

after enlarging, slide back over those lying more caudad so that at least 
the external branchial grooves lie in a pocket called the cervical sinus. 
This sinus is later closed by a process from the hyoid arch which extends 
over it quite as in the development of Anura. Internally the entodermal 
branchial pouches, with the exception of the first, disappear, but the first 
persists as the Eustachian tube and the greater part of the middle ear. 


The swim bladder arises as a diverticulum of the alimentary canal 
remaining in contact with that canal by a pneumatic duct in the ganoids 
and one group of teleosts (Physostomi) (Fig. 441). This duct, although 












Fig. 441. 

Swim-bladders of those fresh-water fish whose air-bladders have 
a duct (physostomous). A, Pickerel; B, Carp; C, Eel. h, swim- 
bladder; d, duct; g, red gland; oe, oesophagus. (From Kingsley after 

usually emptying into the oesophagus, may connect with the stomach. 
However, in most teleosts the duct disappears entirely at an early date. 
The swim bladder lies dorsal to the digestive duct outside of the peri- 
toneum, although below the vertebrae and excretory organs. It may be 
of almost any dimensions, sometimes extending the entire length of the 
body. In some forms of teleosts, which remain almost constantly at the 
bottom, it is entirely absent. The swim bladder, although usually 
unpaired, is paired in most ganoids and may even form three divisions of 
connecting sacs. There may be diverticula of any and all kinds. The 
internal part of it may be smooth and simple, or it may be subdivided by 
various septa, or it may even be alveolar like the lungs of higher 
vertebrates. There may be striated muscle fibers in the walls. In some 
Siluroids and Cyprinoids the walls are even partly calcified because some 
of the vertebral processes are included in the walls. 

The blood supply of the swim bladder is arterial and comes from 
either the aorta or the coeliac axis ; sometimes different portions receive 
blood from both these vessels. The arteries break up in the walls to 
become networks of minute vessels known as retia mirabilia. These 
often form "red spots" on the inner surface. From the retia the blood 
passes to the postcardinal, hepatic, or vertebral body-veins in the ganoids 
and physostomous species, especially in those wnth a wide pneumatic 

The swim bladder contains a greater quantity of Oo than is found 
in solution in the water in which the fish lives. It is therefore probably 
a storage organ for O2 for use when the fish dives to lake bottoms in 

338 Comparative Anatomy 

the summer for food. This can be understood the better when it is 
remembered that there is no O2 at all at the bottoms of lakes in summer. 

The swim bladder is supposed to make it possible for its possessor 
to regulate its equilibrium while in its watery medium. This supposition 
has the following facts upon which to rest its validity : ground-feeding 
teleosts do not have it, but those who must adjust their position in such 
a way as to obtain the requisite food do have it, while in many of these 
there is a diverticulum from it to various portions of the ear. 

In all the higher forms of animals and in some few fishes — dipnoids 
— the lungs arise as an outpushing from the ventral side of the pharynx 
immediately behind the last gill pouch. This outpushing divides almost 
immediately into a right and left half, and just as the outgrowing from 
the digestive tract carried the covering of that tract before it, so, too, a 
peritoneal covering is carried before the respiratory organs. 

As development goes on, the growing part protrudes into the coelom 
so that the parts lying therein have an entodermal lining which was 
derived from the epithelium of the pharynx, while the outer layer of 
peritoneum is serous mesenchyme carrying blood and lymph vessels as 
well as nerve and smooth-muscle fibers between the two. That portion 
of the respiratory system from the pharynx to the lungs consists of 
trachea, bronchi, and their accessories. These together constitute what 
are commonly called air ducts. The lungs are treated as distinct from 

On the ventral side of the trachea, in air-breathing animals, there 
is a separation which forms the larynx (Fig. 442), the beginnings of which 
can be studied in amphibia, in the lower forms of which a simple pair 
of cartilages are developed on the sides of the glottis (the glottis simply 
being an elongated slit connecting the pharynx with the air ducts). These 
cartilages develop in the position of a reduced visceral arch. In other 
forms, such as the Urodeles, the more cephalic ends of the lateral car- 
tilages separate from the rest and form an arytenoid which is the first 
of the laryngeal cartilages, and is imbedded in the walls of the glottis. 
The remaining lateral cartilages may remain as they originally develop 
or divide into any number of pieces. However, the more cephalic pair 
of these pieces often fuse in the mid ventral line to form the cricoid, 
which is the second element of the laryngeal framework. Attached to 
these cartilages there are various antagonistic muscles which make it 
possible to open and close the opening. 

The vocal cords are formed by a pair of folds of the laryngeal lining, 
which extend parallel to the margins of the glottis. Sound is produced 
by the vibrations caused by the air passing over these cords as they are 
relaxed or tightened in different degrees. The larynx is quite rudimen- 
tary in reptiles and birds. In the latter the syrinx, shortly to be 
described, takes the place of the larynx. 

Respiratory System 


In the mammals, one or more thyroid cartilages are added on the 
dorsal side to those already described. In the monotremes, the hyoid 
apparatus and the larynx are most intimately connected, but in the 
higher forms of mammals, such an association is not so intimate even in 
the embryo. 

The thyroid cartilage forms a half ring on the ventral side of the 
anterior end of the larynx in the higher mammals. The anterior dorsal 
angles form cornua which connect with the hyoid by a ligament. Dorsal 

lig ly crit ' 

Fig. 442. 

A, Muscles of larynx (voice box) of Rana esculenfa. Dorsal view, aryt., 
arytenoid cartilage; dil.lar., dilator muscle; hy.lar., hyo-laryngeus muscle; 
lig.i.cric, intercricoideum ligament; s., tendon of posterior sphincter muscle; 
sph.ant., anterior sphincter muscle;, posterior sphincter muscle. 
(After Gaupp.) 

B, Laryngeal apparatus of a Turtle, ar, arytenoid; b^--, first and second 
branchial arches; cr, cricoid; d, dilator laryngis muscle; g, glottis; h, hyoid; 
he, hyoid cornua; sph, sphincter laryngis; tr, trachea. Cartilage is dotted, 
bone is black. (From Kingsley after Goppert.) 

to the thyroid is the glottis with the arytenoids in its walls. Posterior 
to the glottis is the ring-shaped cricoid which is followed by the trachea. 
Anterior to the glottis lies the epiglottis which is a fold of mucous mem- 
brane supported by an internal cartilage which articulates with the 
anterior margin of the thyroid. The epiglottis usually stands erect, thus 
leaving the glottis open during respiration, while during deglutition it is 
pulled back into the glottis, supposedly preventing the entrance of food 
into the trachea, but there are numerous cases on record where the epi- 
glottis has been removed and such individuals seem to have no difficulty 
with their food getting into the ''wrong throat." 

The cavity of the larynx bears a vocal cord internally on either side. 
These are folds of the mucous membrane which extend from the thyroid 
to the arytenoids. By moving these latter cartilages they can be 


Comparative Anatomy 

tightened or relaxed to alter the pitch of the note caused by their vibra- 
tion. A pocket lies anterior to the cords, the laryngeal ventricle (sinus 
of Morgagni), one on each side, quite small in most mammals but w^ell 
developed in the anthropoid apes to large vocal sacs. In the chimpanzee 
there is a median vocal sac in addition. These act as resonators and add 
strength to the voice. 

The larynx is prolonged in w^hales and marsupials so that it projects 
into the choana behind the soft palate. This is an adaptation to the 
manner of taking food from the v^ater and breathing at the same time 
in the v^^hales. In young marsupials the milk is forced into the mouth 
by the muscles of the mammae of the mother; an arrangement that 
prevents strangulation. 

The trachea (Fig. 442) in the higher forms has a series of cartilagi- 
nous rings forming its vv^alls. It varies in length and size as well as in 
the quantity of cartilage which strengthens its walls in the different 
genera. It is, as a rule, shortest in lizards and often convoluted in turtles. 
The cartilaginous rings may be entirely complete or the dorsal part of 
the ring may be of membrane. It is usually longest in birds. 

It is interesting to note that the larynx never forms the voice organs 
of birds. In this form of animal life, the sound producing parts are 
formed from membranes which also vibrate by the passage of air but 
the voice organ is located at the point where the trachea divides into 
bronchi and is known as a syrinx (Fig. 443). The most common form 
of this organ is that in which the last rings of the trachea unite to form 

a resonating chamber, the tym- 
y' sp!b panum, while folds of mem- 

tfJ.J brane, called internal and ex- 

ternal tympanic membranes 
(not to be' confused with the 
similarly named structure in 
the ear), extend into the cavity 
from the median and lateral 
wall of each bronchus. 

In some instances there is 
also an internal skeletal ele- 
ment, called a pessulus, bearing 
a semilunar membrane on its 
lower surface. This type of 
syrinx may be symmetrical 
and may even form a bony 
resonating vesicle. There are 
various muscles attached to 
trachea and bronchi, which per- 
mit an alteration of the tension 
of the folds in all forms of 

Fig. 443. 

Columba livia. The lungs with the posterior end 
of the trachea, ventral aspect., aperture of 
anterior thoracic air-sac; br., principal bronchus; 
fcr' .hr." hr'"., secondary bronchi; p, aperture of ab 
dominal air-sac; p.a., pulmonary artery entering 
lung;, aperture of posterior thoracic air-sac; 
p.v., pulmonary vein leaving lung; sh.b., aperture 
of interclavicular air-sac; sp.b., aperture of cervical 
air sac; sy., syrinx; tr., trachea. (From Parker's 

Respiratory System 341 

the syrinx so as to make possible a change in the sounds uttered. 

In mammals the cartilaginous rings of the trachea are dorsally 
incomplete ; this position being closed by membrane. A structure of this 
kind permits the tube to remain open and yet also permits it to "give" 
a little when food passes down behind it through the oesophagus. 


In the lung fishes there is visually a single sac, although several 
types of these animals have paired lungs. The pulmonary arteries spring 
from the last efferent branchial artery of both sides. The blood supply, 
therefore, under normal conditions is arterial, and the lungs cannot act 
as respiratory organs. In times of drought (Protopterus), or when the 
water is fouled (Ceratodus), the gills no longer function, and the pul- 
monary arteries bring venous blood to the lungs. 

In amphibia the two lungs are elongated. They are united at their 
bases though true bronchi are absent. They may or may not have 
alveoli. In the frog the two lungs are distinct, the walls being divided 
into a series of sacs or infundibula lined with alveoli. The infundibula 
open into a central chamber, which, since it is ciliated and has numerous 
glands in its walls, may be compared to a bronchiole. 

In those terrestrial urodeles which are lungless in all stages of 
development, no traces of larynx or trachea occur at all, even after the 
gills are absorbed. In such species there is a considerable development 
of capillaries in the skin as well as in the walls of the mouth and pharynx, 
so that the respiratory functions are transferred to these parts. In the 
frog, as already shown, the skin is respirator}^ and largely supplied by 
the cutaneous arteries arising from the same arch as the pulmonary 

The air ducts enter the anterior end of the lungs in amphibia, while 
in higher forms the lungs extend anteriorly to the entrance of the 
bronchi on the medial side. This change is in part the result of the 
transfer of the heart into the thorax, due to the position of the pulmonary 
arteries which force the bronchi toward the center of the lungs. In 
amniotes, also, the ducts are characterized by the presence of cartilage 
in their walls, so that they are true bronchi. The bronchi may extend 
inside of the lun^s and divide into secondary and tertiary bronchi. 

The lungs of reptiles are often non-symmetrical ; sometimes one is 
even absent. In the snakes the lungs consist of a single sac lined with 
infundibula either in part or throughout. In the lizards there are one 
or more verticle septa dividing the lung into chambers Hned with alveoli 
while a part of the bronchus may extend to the extremity of the lungs. 
The septa in the chameleons do not reach the distal wall ; consequently, 
the chambers communicate so that the bronchus enters a cavity known 
as the atrium. This connects with the various chambers separated by 
the septa and these in turn open into a terminal vesicle. This whole 

342 Comparative Anatomy 

structure seems to anticipate the parabronchi — the small uniform sized 
air-tubes in the lungs of birds, which connect the larger secondary 
branches of the bronchial tubes. This resemblance is increased by 
the development in these same lizards of long, thin-walled sacs from 
the posterior part of the lungs, which extend among the viscera, even 
into the pelvic region. The air sacs are used to inflate the body. It is 
well to remember and apply what has just been said to the study of simi- 
larly named structures in the bird. In turtles and crocodiles there is no 
atrium and the whole lung has a spongy texture. The bronchus in turtles 
enters on the ventral side of the lung and not as in lizards in the medial. 


The pharyngeal clefts take the form of gill sacs, each of which opens 
into the pharynx in a U-shaped slit, resembling that of Amphioxus, and 
opens to the exterior by a small pore. These gill-slit openings to the 
pharynx are supported by thin, chitinous bars resembling the gill bar 
system of Amphioxus. 


The characteristic respiratory organs of aquatic vertebrates are gills 
or branchiae. Gills are finely divided comb-like outgrowths of the ecto- 
dermal or endodermal epithelium lining the branchial clefts. The num- 
ber of clefts or gill slits vary from five to seven in number. Each cleft 
is separated from its neighbor by branchial septa. The more primitive 
the fish, the larger number of branchial clefts it is likely to have. The 
modern types have regularly five clefts. Heptanchus, sometimes men- 
tioned as the most primitive living species of shark, has seven clefts, 
while Hexanchus, another primitive shark, has six, and elasmobranchs in 
general have five fully developed clefts and a vestigial anterior first cleft 
called a spiracle. 

The spiracle is the rudimentary first cleft, which is also found 
among the most primitive teleostomi (Crossopterygii and Chondrostei). 
It is present in the embryos of Teleostei and Holostei, although here it 
is closed before hatching. In the Halocephali, an aberrant group of 
elasmobranch fishes, the fifth cleft is closed in the adult, which reduces 
the number of functional clefts to four. The cyclostomes have on the 
whole a larger number of clefts than the true fishes. However, the hag- 
fishes (Fig. 366) of the family Myxinidae have no more than six pairs, 
while those of the family Bdellostomidae (Fig. 366) have as many as 
fourteen pairs, and the lampreys all have seven pairs. 

The direction of change in fishes appears to be one of reduction in 
the number of clefts from fifty or more in Amphioxus (Fig. 437) and 
Ascidians (Fig. 313), to fourteen to six in the cyclostomes, seven to five 
in the true fishes, and four in the Holocephali. 

Respiratory System 343 

The openings of the clefts to the exterior differ in different groups 
of fishes. Among the elasmobranchs each cleft usually opens separately 
and is not covered by any flap or operculum, although in Chlameidose- 
lachus, the primitive frilled shark, each cleft has a backwardly directed 
flap or gill cover. The first three clefts in the Holocephali are covered 
by an operculum, and only the fourth, or the last functional, cleft opens 
freely to the outside. In the great majority of teleostomi and in the 
Dipneusti, the five clefts are covered with a flap-like operculum, capable 
of opening and closing, thus effectively protecting the branchial filaments 
from injury. In some of the eels and in other specialized teleosts, the 
gills are completely covered w^ith a fold of skin, the only exit being 
through one or two small water pores. There are two quite different 
and distinct kinds of gills found among fishes, namely: external and 
internal gills. 

External gills are purely larval or embryonic organs. They are not 
functional in any adult fish. Their homologues are found in the perenni- 
branchiate amphibia and are believed to be paedogenetic or permanent 
larval types. External gills are finely branched processes of the ectoder- 
mal epithelium of the branchial tract. They are found in the embryos 
of many elasmobranchs and in some teleosts. A notable case of larval 
gills is seen in the advanced larva of Polypterus (Fig. 368). 

The true functional gills of adult fishes are internal. They are finely 
divided diverticula of the endodermal epithelium of the branchial clefts. 
The nasal cavities are blind sacs which do not communicate with the 
mouth. Such communication begins with amphibia. 


In all of the groups of fishes above the elasmobranchs there is a 
single, or paired, air-bladder (probably homologous with the lungs of 
higher forms), a sac-like diverticulum of the pharynx, derived from 
either dorsal or ventral sides of the alimentary tract. It is in all cases 
supplied with blood from the pulmonary artery (which, in turn, arises 
from the last" efferent artery of either side), and, primitively at least, 
subserves two functions: (1) that of a hydrostatic or buoyancy organ, 
and (2) that of an accessory respiratory organ or primitive lung. In 
the most primitive teleostome fishes, the Crossopterygii, it is used as a 
lung when the water is foul ; in Amia, it is constantly functional as an 
air-breathing apparatus; while in the Dipneusti (lung-fishes), it is an 
elaborately pouched lung used to tide the fish over a period of drought. 

In certain other fishes that have acquired terrestrial habits, such 
as the climbing perch, Anabas (Fig. 371), which will drown if immersed 
in water, and the air-breathing eel, Clarias, there is an extensive post- 
branchial chamber, provided with labyrinthine, or arborescent, elabora- 

344 Comparative Anatomy 

tions of the epithelium that are highly vascular and play a pulmonary 


Here branchial respiration is carried on in the six pairs of branchial 
clefts. These branchi are primitive respiratory organs, consisting of 
mere diverticula of mucus membrane, richly vascular, and supported by 
cartilaginous processes called gill-rays. The w^ater enters the mouth 
and is forced out through the gill slits. In doing so it aerates the gill 
filaments, and provides oxygen for the blood v^hich circulates rapidly 
through them. 


External gills are found in the perennibranchiate urodeles (Fig. 
374) throughout life and in practically all amphibians v^hile in the larval 

The epithelium covering these external gills is ectodermal so that 
they are really cutaneous and not pharyngeal gills. They are, therefore, 
of a totally different nature from the so-called external gills of the 
embryos of Elasmobranchs and Holocephali, in which case the external 
gills are only filaments of the internal gills prolonged through the 
branchial openings. 

Internal gills develop only in the larvae of Anura and are probably 
homologous with the internal gills of fishes although even here the 
epithelium may be ectodermal. In many species of Salamanders, lungs 
are absent, but in most amphibians, they develop as ventral outgrowths 
from the oesophagus. The left is usually the longer. The lungs are 
united at their base, although true bronchi are absent. In the lungless 
Salamanders respiration is exclusively cutaneous and pharyngeal. The 
lungs are supposed to have secondarily disappeared in these animals. 
The air ducts enter the anterior end of the lungs in amphibia, while in 
amniotes the lungs extend cephalad to the entrance of the bronchi which 
is on the medial side. This change is due to the transfer of the heart 
into the thorax so that the pulmonary arteries then force the bronchi 
toward the center of the lungs. The ducts in the amniotes have cartilage 
in their walls; they are thus true bronchi. These bronchi often extend 
into the lungs where they divide into secondary and tertiary bronchi. 


Gills are absent and gill-slits disappear in all animals higher than 
Urodeles. The lungs are large and complicated and often non-symmet- 
rical; sometimes one is lacking. In the snakes, the lungs consist 
of a single sac lined with infundibula either in part or throughout. In 
the lizards there are one or more vertical septa dividing the lung into 
chambers lined with alveoli, while a part of the bronchus may extend 

Respiratory System 345 

to the extremity of the lungs. In the chameleons, the septa do not reach 
the distal wall ; consequently, the chambers communicate so that the 
bronchus enters a cavity known as the atrium. This connects with the 
various chambers separated by the septa, and these in turn open into a 
terminal vesicle. This whole structure seems to anticipate the 
parabronchi — the small uniform sized air-tubes in the lungs of birds — 
which connect the larger secondary branches of the bronchial tubes. 
There develop in these lizards long, thin-walled air-sacs from the 
caudal portion of the lung. These extend among the viscera even into 
the pelvic region. The air sacs are used to inflate the body. In turtles 
and crocodiles there is no atrium, and the whole lung has a spongy 
texture. The bronchus in turtles enters on the ventral side of the lung 
and not as in lizards on the medial. 

Inhalation and exhalation are effected partly by drawing in the neck 
and thrusting it out agam so as to decrease and increase the volume 
of the thoracic cavity. The air is also swallowed into the lungs by filling 
and then emptying the throat. 


Birds have large lungs each of which possesses nine small air-sacs. 
The air enters the bronchi and passes to the air-sacs. The air is thus 
warmed before being taken into the alveoli of the lungs. It makes its 
exit through the excurrent bronchi. A complete change of air occurs at 
each inspiration and expiration. The trachea and the larger bronchi are 
kept open by means of rings of cartilage; the trachea is enlarged, just 
before it divides, into a syrinx or voice box (Fig. 443), a structure limited 
to birds, in no way homologous with the larynx of mammals. The 
mechanics of voice production in birds depends upon forcing the air 
through a flexible valve which is set into vibration. The lungs also 
connect with visceral air-sacs and with air-spaces in the bones. 


There are two points of view regarding the relation of mammalian 
lungs to the respiratory apparatus of the lower forms of animals. 
One view holds that the lungs are merely a further development of the 
air-bladder of fishes; the other insists that they are more likely to be 
modified gill-pouches which have grown caudally into the coelom instead 
of opening to the exterior by growing laterally. 

The fact, however, that the pneumatic duct is dorsal in position and 
the blood supply is arterial makes the first view seem improbable. The 
latter view is supported by the fact that the lungs are paired outgrowths 
from the pharynx immediately caudal to the last gill clefts and in serial 
order with them. The blood supply from the sixth arterial arch would 
be in full accord with this view. Then, too, both in the earlier stages 

346 Comparative Anatomy 

and in the primitive forms, the skeletal support of larynx and trachea 
has the relations and appearance of rudimentary gill-arches, while the 
muscles surrounding this region are modified from those of the visceral 

Each lung is enclosed by a pleural membrane and the pleural cavity, 
in v^hich it lies, is cut off entirely from the rest of the coelom by 
the muscular diaphragm. This muscle usually lies transverse to the 
main axis of the body. It is attached close to the inner margin of the 
lower ribs and extends headward as a sort of tent or dome. The lungs 
may be divided into lobes and lobules. The right one usually has the 
greater number. In whales, elephants, and odd-toed ungulates there 
may be no lobules at all. In the monotremes only the right lung has 

From the main bronchial tube there are dorsal and ventral sec- 
ondary bronchi ; the ventral redivide. When the more cephalic bronchi 
lie in front of, or above, the pulmonary artery, they are called eparterial 
bronchi while the others are known as hyparterial. 

Respiration is made up of inspiration and expiration. This has 
already been described in the study of the frog. Little is known regard- 
ing this process in turtles and other reptiles. In birds, the lungs are 
definitely attached to the ribs and vertebrae so that with every motion 
there is both a change in shape and size. 

In the mammals, the ribs lie at an oblique angle to the vertebral 
column. As the intercostal muscles are contracted and relaxed, the ribs 
turn slightly and can increase and diminish the size of the thoracic 

The diaphragm forms a complete partition between the thoracic and 
abdominal cavities and aids materially in respiration as it flattens when 
contracted. This increases the size of the pleural cavity and draws in air 
through the trachea. The abdominal muscles likewise play a part. 
Expiration is caused in part by the action of the intercostal and abdo- 
minal muscles and in part by the elastic tissue and smooth muscles in 
the lungs themselves. 


It will be recalled that the entire respiratory tract grows from the 
primitive digestive tract. It is, therefore, not difficult to understand 
that there are certain fishes which use a caudal portion of the diges- 
tive tract for respiration. In Cobitis, water is drawn in and expelled 
through the anal opening. The more caudal end of the digestive canal 
is very vascular and is used in respiration. 

Among the amniotes, the lungs are not functional either before 
hatching or before birth. Still, oxygen is necessary for the development 
of the embryo and the carbon dioxide which has formed must have an 

Respiratory System 

outlet. The used for ^;<^^^i^ft^i:::^ ^S 
function is called the «^^^/S^,hfdgesSe canal which has already 
lum from the more <=^"d^^' P^^^Vryology. It becomes larger with the 
been studied m °- -° ^"'^^f^.^^^Slar and is absorbed in some 
growmg embryo. It '^ ^^ ""j/j.^^n off with the placenta m mam- 
forms such as the Sauropsida or is drawn bladder, 
mals The basal part, however, persists as the urinary 



TO understand the modern interpretation of the circulatory system, 
it is necessary to have clearly in mind what is called the prob- 
able ancestral condition of this system in the lower forms of 
animals. Thus one may observe how in each of the succeeding higher 
forms something is added to the development of the animal of the next 
succeeding scale below. 

Some have thought that the original circulation consisted of a 
lymphoidal liquid alone and, then, as time went by, this type of circula- 
tion specialized into what we now term a blood circulation. It is thus 
supposed that the lymph vessels, as we find them in modern forms of 
living animals, furnish a clue as to how the primitive systems of vessels 
appeared. It is all quite speculative, however. Another explanation, 
which has more plausibility in its favor, is that the main blood vessels 
are the remnants of the segmentation cavity which have become oblit- 
erated by the growth of mesoderm ; the part not obliterated then became 
the blood vessels. 

In any explanation that is built upon the Haeckelian ''law" of bio- 
genesis there not only remains much to be explained but various occur- 
rences even must be explained away. In this theory, it is supposed that 
much of the race history has been lost in development, while a develop- 
ment of additional vessels of various kinds has covered up some of the 
older developmental processes. 

Many blood vessels, which should arise as fissures between other 
tissues, are found to be formed as solid cords of cells. These may later 
form a lumen and be converted into tubes ; in other instances vessels 
which originate separately in the embryo may fuse together during 
development to form a single one. 

There are various main points, however, which must be understood 
in any discussion of the blood system (Fig. 444, A, B). There is a 
dorsal tube carrying the blood toward the tail. From this tube various 
vessels extend toward the right and left at almost right angles through 
the dorsal tube. Those that pass toward the outer side of the animal are 
called somatic, while those that pass toward the inner region are called 
splanchnic vessels. These transverse vessels connect with two ventral, 
longitudinal tubes, one of which is in the wall of the digestive tract 
which runs headward and unites with the other one which has passed 
through the ventral body wall, so that, after the union of the two, a 
single tube is found coursing to the head end of the body. In one of the 
lowest forms of chordates, namely, the Amphioxus, various parts of this 
system develop muscle walls and then act as pumping organs. 

Circulatory System 



Comparative Anatomy 

Poit Iransverse 
Dorsal romui 
of dorja.1 
inter Stcment. 
. al OJ-t^ry 

Yentro- ^ 
I artery 

ILat. ramus 
div dorsal 

tost- cosTclI anastomosis 
o f dorsal ramus of dcnal 
■mtirJefmsnfa.i aj-fei-y 

Dorsa.1 intersegmental 

JJcrsal aorta. 

Ventral splanchr 

Ventral anastomosis cF 

ventral ctiv of dorsa.1 mtersegmenftzl artery 

Fig. 444. 

Comparisons of Circulatory systems. A, a diagram of the vascular system of 
Amphioxus, from the right side. a.b.a., Afferent branchial arteries; ar., carotid 
continuations of the suprabranchial arteries; cons.s., contractile swellings on the 
afferent branchial arteries;, dorsal aorta; a., efferent branchial arteries; 
hep.v., hepatic vein; int., intestine; Ir., liver; m.v., "moniliform" vessel from left 
carotid;, nephridial plexus; ph., pharynx; s.i.v., subintestinal vein; sbr.a., 
suprabranchial arteries; tr.y., transverse vessel joining the carotids;, ventral 
aorta;, vessels of cirri;, vessels of synapticulae ; v.t.b., vessels of tongue- 

B, A diagram of the arterial system of a dogfish, seen from the right side. 
a.h.a., Afferent branchial arteries; a.mes., anterior mesenteric artery; c.c, common 
carotid artery; cd., caudal artery; coel.a., cceliac artery;, dorsal aorta; e.b.a., 
efferent branchial arteries; e.c, external carotid artery; epibr., epibranchial 
arteries; ht, heart; hep., hepatic artery; hy.a., hyoidean artery; (this joins the 
internal carotid of the opposite side, which is not shown) ; i.e., internal carotid 
artery; il.a., iliac artery; In.g., lienogastric artery; p.c, posterior carotid artery; 
p.mes., posterior mesenteric artery; ren., renal arteries; scl.a., subclavian artery;, ventral aorta. (From Borradaile.) 

C and D, Comparison of the venous systems of the dogfish and a teleost. anas, 
anastomosis between the two posterior cardinals; ao.v., ventral aorta; atr., atrium; 
brach, brachial vein; card.ant. and card. post., anterior and posterior cardinals; 
caud, caudal vein; cl, ploacal vein;, cpnus arteriosus; duct.cuv., duct of 
Cuvier; hep, hepatic vein; hy, hyoid vein; il, iliac vein; jug.inf., inferior jugular 
vein; lat, lateral vein; leb, liVer; mand., mandibular vein; n., kidney; port., hepatic 
portal vein; port. ren., renal portal vein; segm., segmental veins;, subintestinal 
vein; sin.hy., hyoid sinus; into which the veins from the mandibular and hyoid arches 
open; sin. orb., orbital sinus; sin.ven., sinus venosus; sperm, spermatic vein; subsc., 
subscapularis vein; subcl., subclavian vein;, veins of the swim-bladder; 
ventr., ventricle. (Both figures from Boulenger, A after Parker.) 

E, A diagram of the venous system of the dogfish, a.c.s.. Anterior cardinal 
sinus; c.v., caudal vein; d.C, ductus Cuvieri; h.p.v., hepatic portal vein; h.s., 
hepatic sinus; hy.s., hyoidean sinus; i.j.s., inferior jugular sinus; i.o.s., interorbital 
sinus; U.S., iliac sinus; int., intestine; k., kidney; lat.s., lateral sinus; Ir., liver; 
n.s., nasal sinus; or.s., orbital sinus; p.c.s., posterior cardinal sinus; r.p.v., renal 
portal vein; s.v., sinus venosus; scl.s., subclavian sinus. (From Borradaile.) 

F, A diagram of the principal arteries _ and veins of a pigeon, ao.. Aortic 
arch; Br. a., brachial artery; Br. v., brachial vein; C, carotid artery; cm., 
coccygeo-mesenteric vein; d.a., dorsal aorta; F, femoral vein adjoining femoral 
artery; h.v., hepatic veins; il., internal iliac artery and vein; i.v.c., inferior vena 
cava; ;., jugular vein; La., left auricle; P., right pulmonary artery; Pc.a., pectoral 
artery; Pc.v., pectoral vein; ra., right auricle; rp., hypogastric vein; rv., renal 
vein; sc., sciatic artery and vein. Near the apex of the ventricle the coeliac and 
anterior mesenteric arteries and the epigastric vein are shown, but not lettered. At 
the hinder end of the figure the caudal and posterior mesenteric vessels are shown, 
but not lettered. 

G, The circulatory system of the rabbit, (a) Letters to right — e.c.. External 
carotid; i.e., internal carotid; e.j., external jugular; scl.a., subclavian artery; 
scl.v., subclavian vein; p.a., pulmonary artery (cut short); p.v., pulmonary vein; 
L.A., left auricle; L.V., left ventricle;, dorsal aorta; h.v., hepatic veins; c, 
coeliac artery; a.m., anterior mesenteric; s.r.b., suprarenal body; l.r.a., left renal 
artery; l.r.v., left renal vein; K, kidney: p.m., posterior mesenteric artery (incor- 
rectly shown as if paired); spm., spermatic arteries and veins;, common iliac 
artery. (&) Letters to left; — p.f. and a.f., posterior and anterior facial; e.j., external 
jugular vein; i.j., internal jugular; R.Scl., right subclavian artery; S.V.C, superior 
vena cava; R.A., right auricle; R.V., right ventricle; I.V.C, inferior vena cava; 
r.r.a., right renal artery; r.r.v., right renal vein; s.r.b., suprarenal body; spm., 
spermatic arteries and veins; il., ilio-lumbar vein; f.v., femoral or external iliac vein;, internal iliac veins. (From Thomson.) 

H, Diagram of intercostal (intersegmental) arteries. 

Circulatory System 351 

In all vertebrates the heart lies on the ventral side of the digestive 
tract covered by a pericardial sac. This sac is really a part of the 
coelomic lining. The various large blood vessels, carrying blood frorn 
the heart to the general system, are known as aortae and the large veins 
returning the blood directly into the heart are usually called venae cavae. 

The ventral aorta gives off various pairs of vessels called the aortic 
arches which are situated on each side of the pharynx in the grooves 
called the gill septa. These arches run from the ventral aorta around 
the digestive canal to the dorsal side where they unite to form a long- 
tudinal canal. That is, the arches along each side form a separate canal 
at first; then the two canals unite to form the dorsal aorta, which runs 
caudad the entire length of the body. There may be, and usually are, 
various small arteries arising from any or all of these arterial arches. 
It is necessary that the student know what becomes of the aortic arches 
and in what groups of animals certain ones disappear and others remain 
functional. The first pair of arches lying toward the head end give rise 
to both the internal carotid artery which goes to the brain, and the 
external carotid supplying the more superficial portion of the head. The 
arteries which arise from the dorsal aorta are either somatic or splanch- 
nic, that is, either supply outer or internal portions of the body. Exam- 
ples of somatic blood vessels are the intercostal (intersegmental) arteries 
running between the ribs. The mesenteric arteries, which are distributed 
primarily to the alimentary canal, are of the splanchnic type. 

The subclavian artery, which supplies the arms of the animal, and 
the iliac artery, which supplies the hind limbs, are some of the larger 
and more common of the somatic arteries. 

The splanchnic or visceral arteries do not show much trace of seg- 
mentation. They are distributed to the walls of the digestive tract. Two 
pairs, however, of these vessels are of special importance, namely, a pair 
of omphalomesenteric arteries in front and a pair of hypogastric 
arteries (internal iliac) near the origin of the iliac arteries (Fig. 450). 

There are really no end arteries or veins. All arteries carry blood 
to certain parts of the body through minute capillaries which then 
anastamose with the venous capillaries which drain the various parts 
the arteries supply. 

The head is drained by a pair of jugular veins easily seen above 
the mouth. In fishes there is also a pair of inferior jugulars in the 
region of the lower jaw and the lower side of the gill arches. These 
veins run caudad to the level of the sinus venosus where they are joined 
by a post-cardinal coming from the excretory organs. The jugular and 
post-cardinal on each side unite to form a trunk which runs transversely 
and empties into the sinus venosus. This is called the Cuvierian duct. 
A pair of omphalomesenteric veins enter the sinus venosus from the 
caudal side. These are continuations of a subintestinal , vein running 

352 Comparative Anatomy 

alongside the liver after having passed along the ventral side of the 
digestive tract. This subintestinal vein forms a loop around the anal 
opening and extends to the end of the tail as the caudal vein. The sub- 
clavian vein from the arm may empty either into the jugulars or the post-, 
cardinal near the Cuvierian duct. The blood from the hind limb leaves 
by an iliac vein on each side and runs forward on the lateral body wall. 
It is called the lateral abdominal vein. This also enters the Cuvierian 

Omphalomesenteric and subintestinal veins belong to the visceral 
or splanchnic group. The others are somatic. The vessels mentioned 
are important and should be known thoroughly because they develop 
very early in the embryo, and, practically all later developments as well 
as modifications that take place in them, can only be discussed intelli- 
gently when the basic structures just mentioned are known. There is 
probably no more variable system in the body (even in the same species) 
than the vascular. 


The heart itself is a muscle with a distinctive cellular structure. 
This cellular structure is a sort of "cross" between voluntary and invol- 
untary muscle. The muscle fibers are striated but run in a syncitial 
form (Fig. 23, Vol. I). The muscular walls of the heart itself are known 
as myocardium. The inner layer of the heart, corresponding to the endo- 
thelium of the blood vessels and continuous with them, is called endo- 
cardium, while the covering of the heart is known as the pericardium. 
Lying between the myocardium and the pericardium is a serous liquid 
called pericardial fluid. 

We have already discussed a part of the embryonic method of heart 
development, but it is necessary here to enter into more detail. The 
lateral plates of the walls of the coelom grow centrally beneath the 
digestive canal. There are four regions discernible in these lateral 
plates, namely : the splanchnic or visceral, the mesenterial and somatic 
walls, and the coelomic cavity. 

Between the coelomic walls and the endoderm, one may observe 
various cells called vascular cells. It is supposed that they find their 
origin from the mesothelium. Those that lie most dorsalward assist in 
forming the dorsal blood vessels while those lying ventrad contribute 
to the heart and the ventral trunks. The two lateral plates just men- 
tioned continue until they meet in a ventrad region. This forms the 
ventral mesocardium (Fig. 344). A little later the dorsal region comes 
together to form the dorsal mesocardium so, that now, that which was 
formerly a groove has become a definite tube. The ventral fusion has 
disappeared so as to leave the dorsal part attached; this causes the two 
coelomic cavities to unite and form the pericardial cavity. 

Circulatory System 353 

In turtles and crocodiles there is a small portion of this ventral 
mesocardium remaining, which connects the apex of the heart to the 
pericardial wall. The walls of this tube are now called the myoepicardial 
mantle, and the vascular cells enclosed within this mantle form a con- 
tinuous sheet and become the endocardium or lining of the heart. 

There are still some vascular cells cephalad and caudad to this 
tubular heart. These furnish a lining for the blood vessels which arise 
from the edges of the lateral plates and connect with the heart. The 
first vessels toward the head end (the anterior pair) become the man- 
dibular arteries, while those lying caudad to the heart (the posterior) 
are called the omphalo-mesenteric veins. 

It is at this time also that immediately cephalad to the omphalo- 
mesenteric vein a transverse tube appears on each side connecting with 
the heart tube. These tubes are the ducts of Cuvier. The ridge, where 
the Cuvierian ducts grow, becomes larger until it forms a transverse 
partition known as the septum transversum (Fig. 348). It is this septum, 
or partition, which separates the heart cavity or pericardial region from 
the abdominal or peritoneal cavity. In the myxinoids and elasmobranchs 
this septum never completely closes dorsally but leaves one or two 
openings known as the pericardio-peritoneal canal. 

Where the early embryo is closely appressed to the very large yolk 
sac, as in the bony fishes and in all amniotes, the development of the 
heart is modified. The pharynx is not complete below at first but com- 
municates ventrally with the yolk. The two hypomeres are thus pre- 
vented, for a time, from meeting ventrally. Each hypomere, however, 
is accompanied by its vascular cells. Its edge becomes grooved while 
the grooves are rolled into a pair of tubes lined with endocardium. The 
anlage of the heart consists of two vessels (Fig. 283) for a time, each 
connected in front and behind with its own mandibular artery and 
omphalo-mesenteric vein and surrounded with its pericardial sac. Later 
the two tubes approach and fuse. The formation of mesocardia taking 
place as before, the mesocardia soon disappear and the whole appears 
much as in the small-yolked forms. 

The pericardium is relatively large at first, but in adult forms it 
is usually quite close fitting to the heart when the heart is expanded. 

It must not be forgotten that in systole the heart contracts and 
becomes considerably smaller than normal and that in diastole it expands 
and attains its full size, filling the pericardium accordingly. 

It can readily be understood that, so long as the mesocardia are 
present, the heart tube will be a straight canal connected with the 
pericardial sac in front and behind. However, as the mesocardia entirely 
disappear in due time and the heart tube continues to grow, it bends 
upon itself, something like a capital letter *'S," the bending or flexure 
being largely in a vertical plane (Fig. 283). 


Comparative Anatomy 

At the middle point of the bend the tube remains quite small and 
here is formed what is called the atrio-ventricular canal (Fig. 445). It 
is in front and behind of this canal that the walls become thickened and 
the lumen enlarged. The caudal end, which is also the dorsal in this 
case, forms the chambers known as the atrium or auricle; the ventral 
end becomes the ventricle. Caudad in the atrium there is a constriction 
forming a second chamber called the sinus venosus. It is into this 
chamber that the Cuvierian ducts and the omphalo-mesenteric veins 
enter. The ventral parts of the heart-tubes also form a smaller trunk, 
called the truncus arteriosus, while the ventral aorta connects this por- 
tion of the heart with the mandibular arteries already mentioned. 

While the heart is really a muscle, or rather many interwound 
bundles of muscles, there are certain parts, such as the sinus venosus, 

in which the muscle cells 
themselves are somewhat 
scanty as compared with 
other parts of the heart. 

The endocardium devel- 
ops folds, or valves (Fig. 
445), in certain places so 
that blood may flow forward 
but not backward, and this 
valvular part of the truncus 
is knoAvn as the conus arte- 
riosus. In the vertebrates, 
this conus is reduced to a 
single row of valves with 
the exception of the elasmo- 
branchs, ganoids, and am- 
phibia. The valves lie be- 
tween the auricle and the 
ventricle and are prevented 
from being pushed up into 

PcUnonajy asfttry 

Fig. 445. 

a and h. Reduction of the bulbo-ventricular fold of 
the heart. Ao, aortic bulb; Au, atrium; B, bulbus cordis; 
RV , right ventricle; LV , left ventricle; P (in &) pul- 
monary artery. (After Keith.) 

A, B, C, D, Scheme showing division of bulbus cordis 
and its thickenings into aorta and pulmonary artery with 
their valves. The division begins in B, the lateral thicken- 
ings dividing respectively into a, e, and c.f. Rotation from 
right to left shown in D. (After Heisler.) 

the auricle (when the heart contracts and immense pressure is brought 
to bear upon them) by little ligaments, called chordae tendineae, which 
extend from the edges of the valve to the opposite wall of the ventricle. 
They are kept taut during systole by capillary muscles, called columnae 
carneae. There is also a valve between the auricle and the sinus in some 
vertebrates where the hepatic vein enters into the sinus. 

If the conus arteriosus is followed by a strong muscular region this 
is called the bulbus arteriosus. The bulbus is composed of regular heart 
muscle while the truncus is composed of muscles like the rest of the 
blood vessels. It is for this reason that both conus and bulbus are 
regarded as a part of the heart while anything cephalad to these is con- 
sidered a part of the ventral aorta. 

Circulatory System 355 


After food has been taken into the digestive tract and digested and 
the little villi of the small intestines have absorbed the semi-liquid food, 
this newly absorbed food is ready to become a part of the blood. An 
elaborate system of blood vessels v^ith a wonderfully intricate and elab- 
orate pumping apparatus — the heart — carries this nourishment to every 
part of the body. 

Before taking up the development of this system, known variously 
as the circulatory, or vascular, system, it is necessary that the student 
understand quite thoroughly what the adult organs are like and what 
their function is. Only then may one validly attempt to ascertain how 
and why the organs are placed where they are and how and why their 
function is what it is. The central part of the vascular system is the 
heart. In the mammal, this consists of four definite chambers — two 
auricles at the broad end of the heart, and two ventricles toward the 
lower or apex region. The structure of the heart itself is muscular. 
The compartments of the heart and the work they do belong to the 
circulatory system proper and will be described here. 

Every blood vessel leaving the heart, no matter whether it carries 
arterial or venous blood, is called an artery, and every blood vessel 
entering the heart is called a vein. This distinction must be kept very 

Then, too, it must never be forgotten that blood entering the heart 
through a vein always enters a sinus or auricle. This auricle acts as 
a reception-chamber for all blood entering the organ. After the blood 
has entered this chamber, it passes downward through an opening into 
one of the ventricles, and it is from the ventricle that the blood leaves 
the heart. 

In the higher forms of mammals, such as man (Fig. 445), blood 
enters the right auricle through the large venae cavae, and then passes 
downward through the auricular-ventricular opening into the right 
ventricle. From here it passes through the pulmonary artery to the 
lungs to be aerated (that is, to be thoroughly mixed with oxygen and 
to lose the carbon dioxide that it has gathered in draining the entire 
body). After being aerated, the blood passes back to the left side of the 
heart through the pulmonary vein to enter the left auricle, and then 
passes down through a left auricular-ventricular opening into the ven- 
tricle. From the ventricle the blood stream is sent forth through the 
aorta to all parts of the body, supplying the parts with food and nour- 

The system just described is known as the systemic because the 
blood which leaves the heart through the aorta nourishes all parts of 
the body. 

The arteries break up into smaller arterioles and capillaries. The 

356 Comparative Anatomy 

liquid part of the blood is called blood-plasma so long as it is contained 
within the blood vessels, and lymph as soon as it has seeped through 
the walls of the blood vessels and bathed the surrounding tissues. It is 
gathered up from here by the various lymphatic vessels which unite to 
form the large lymph duct. This duct empties into one of the veins of 
the neck. The blood, which has remained within the blood vessels and 
passed through the capillaries, is taken up by the venous capillaries and 
passes toward the heart either directly or indirectly through a portal 

It is essential that one appreciate that the arteries supply all parts 
of the body with nourishment and that the veins do the draining. It 
follows, then, that the arteries begin as vessels of some size and become 
smaller and smaller as the blood supply from the heart becomes dis- 
tributed more and more, while veins begin as capillaries and continu- 
ally increase in size. An artery and a vein often lie side by side, but 
the blood current in the vessels is running in an opposite direction. 

In addition to the systemic circulation there is also the pulmonary 
circulation, which is the name given to the blood stream leaving the 
right ventricle of the heart, passing through the pulmonary arteries to 
the lungs, and after being aerated, returning through the pulmonary 
veins to the left auricle, from whence it flows downward into the left 
ventricle to be ready again for the systemic circulation. 

Whenever a vein splits up into capillaries so that the venous blood 
must pass through an organ on its way back to the heart (either to have 
waste substances removed, as in the kidneys, or to take up new sub- 
stances as in the liver), and this blood is then again collected by venous 
capillaries and sent on its way, a portal system is formed. 

The renal-portal system and the hepato-portal system are the two 
important ones in the economy of man's body. 

When the circulatory system of the frog was discussed, it was 
stated that one must not forget that the material with which the heart 
works is blood, but that the heart is similar to a pump or an engine, 
and, that, consequently, just as a pump or an engine which is used for 
the purpose of forcing water through a great hydraulic system requires 
water in two places and in two ways to continue its work, so the heart 
requires blood in two places and in two ways to do its work. 

The engine requires water in its boiler so that steam can be pro- 
duced. This steam then supplies the force for its work of pumping 
water, let us say, through the water-pipes of the building in which it 
is installed. So, too, the heart must have a blood supply to furnish it 
with energy just as the engine requires water to manufacture its steam. 
Therefore, there are blood vessels running into the heart-walls and into 
the walls of blood vessels themselves so as to furnish these with material 
to produce the required energy to continue their pumping power. The 
blood vessels that supply the heart walls are known as coronary vessels. 

Circulatory System 357 

The coronary arteries leave the aorta immediately after the aorta, in 
turn, has left the left ventricle. The blood vessels in the walls of blood 
vessels are known as vasa-vasorum. 

It is essential that these two systems be kept separate and distinct. 
The mere pouring- of blood into the cavities of the heart is equivalent 
to the water in the tank of an hydraulic system, while the blood which 
enters the heart muscle itself is equivalent to the water in the engine's 
own boiler that furnishes the steam from which the energy, in turn, 
comes to make pumping possible. 

This analogy may be carried a little further; for, just as the water 
in the hydraulic system, if it be used for drinking purposes, must be 
filtered, so, before the blood, which is pumped through the vascular 
system, can be used again, it must likewise be filtered. This is the work 
of the portal systems. 

A final point to be borne in mind, before taking up the circulatory 
system in detail, is that the vertebrate circulatory system is known as 
a closed circulation, as distinguished from the open system seen in some 
of the lower forms of animals, such as the crayfish. 

What is meant by a closed system, is that the blood from the time 
it leaves the heart until it returns, is always in direct communication 
by means of arteries, capillaries, and veins. There are no open spaces 
through which the blood can pass out of these vessels. A seeming 
exception is the lymph. This does not pass through an opening, how- 
ever, but seeps directly through the walls and bathes all parts of the 
inter-capillary region. 


It is well known that the mammalian heart has its point, or apex, 
to the left, but the student must know how this has come about. He 
must also know why it is that, just as with the digestive tract, certain 
nerves which lie in the right and left side of the early embryo, come to 
lie on the dorsal and ventral sides in the adult. The heart, like the 
digestive tract, grows something on the order of a straight tube, although 
made up of separate cephalic and caudal ends which have become fused 
together. As the embryo continues developing, the heart turns to the 
left, so that the nerves, which lie upon the right side, will now be ventral, 
and those which lie upon the left side will be dorsal, while the right 
auricle and ventricle, which have been brought ventral by this turning, 
to the left now occupy almost the entire ventral portion of the heart, 
the left auricle and ventricle being dorsal. It is for this reason that but 
a very small portion of the left auricle and ventricle can be seen from 
the ventral side of the body. It will be remembered that in the embryo 
of the chick mention was made of mesoblastic cells which Avere derived 
from three separate sources. One source of these is from the primitive 
streak. The second source is from that scattered group of cells left 

358 Comparative Anatomy 

between the ectoblast and entoblast when the entoblast became a dis- 
tinct layer of cells. Thirdly, in the middle and lateral parts of the area 
pellucida, cells are budded off from the upper side of the entoblast to 
become mesoblast, at about the time the primitive streak is forming. 

Now, all of these mesoblastic cells together unite to form a con- 
tinuous layer. This layer continues expanding until it passes beyond 
the boundaries of the area pellucida and forms a middle layer in the 
inner zone of the area opaca. This zone is the vascular area (Fig. 264). 
It is in this area that the blood vessels begin to form. This occurs in the 
chick on the very first day. A network appears in the entire vascular 
area which surrounds the embryo. Here irregular reddish blotches are 
formed, called blood islands ; it is from these blood islands that the red 
corpuscles are formed. This network develops into a system of cords, 
at first solid ; but soon a lumen is acquired and, as the vessels unite, a 
continuous, but indefinite, blood vessel is formed. The very first vessel 
which becomes definitely shaped so that it can be recognized as a part 
of the vascular system is formed around the entire vascular area as a 
sort of boundary and is called the sinus terminalis (Fig. 284, C). 

The blood islands appear in cross section as little local thickenings 
on the dorsal walls of the blood vessels. These bud off into the cavities 
of the vessels and form the first blood corpuscles, and it is supposed 
that from these all the colored corpuscles of the blood are descended. 

The network of vessels continues to grow, some of the vessels later 
becoming arteries, some veins, and still others remaining small as capil- 
laries. These unite and extend toward the embryo, while, within the 
embryo proper, there has been a growth of the vascular system also, 
which has extended outward toward this vascular area. All these ves- 
sels unite to form the entire vascular system. 

Larger vessels of the vascular area unite with the posterior end of 
the heart which by this time has already begun to beat. The other 
vessels unite with the anterior, or cephalad, end of the heart and become 
the arterial system, so that by the end of the second day, in the chick, 
a complete vascular system has already been formed with a beating 

At first the heart consists of only two longitudinal vessels Avhich 
are connected at the cephalic end. These spread out caudad like an 
inverted ''V" (Fig. 283). The arms of this ''V" shaped portion soon 
fuse together and look like an inverted ''Y." The cavities of these two 
fusing tubes remain apart for some time and then form one cavity. 
That is, the endothelial lining remains separate as tv/o distinct cavities 
for a time, even after the muscular walls have united. 

On the dorsal surface, the muscular walls are incomplete also for a 
short time, but after complete fusion the walls also are completed. It 
is the stem of this "Y" which forms the heart. The two diverging arms 
of the "Y" unite, or rather have united some time before this, so that 

Circulatory System 


they are continuous with the large vitelline veins which bring the blood 
back to the heart from the vascular area. 

The heart is now a short straight tube attached to the ventral wall 
of the pharynx and consists of the muscular united part of this '*Y," 
the two arms are the ends of the diverging vitelline veins which run 
backward or caudad at the hindermost angle of the head fold. As this 
fold is pushed farther and farther back, the straight part of the *'Y" is 
naturally pushed back also and lengthened. Not only this, but this 
straight part of the heart grows more rapidly than does the place to 
which it is attached, so that it does not even find room enough to con- 
tinue its growth with the heartfold, but must bend into a loop with its 
convexity toward the right side of the embryo. The heart has now lost 
its attachment to the pharynx (with exception of its two ends). The 

Fig. 446. 

/, Schematic longitudinal sections of the heart. A, dogfish, B, Ganoids, and C, 
Teolosts. a, atrium; b, bulbus arteriosus (an enlarged portion of the truncus 
arteriosus); c, conus arteriosus; k, valves; s, sinus venosus; t, truncus arteriosus; 
V, ventricle. (After Boas.) 

II, The circulatory system in the amphibians. A, Urodele, and B, Anura. 
a.l.and a.r., left and right atria; ao.w., aortic root (radice) ; ca, carotid arteries, 
which spring from the conus arteriosus together with the ao.w.; La., pulmonary 
arteries which carry venous blood from the ventricle to the _ lungs ;_ l.v., pulmonary 
veins which carry arterial blood to the left atrium; v, the veins which carry venous 
blood from the general body system to the right atrium; ventr., ventricle. (From 
Schimkewitsch after Wiedersheim.) 

caudal end of the heart, in which the vitelline veins empty, is called the 
venous and the cephalic end is spoken of as the arterial end of the heart. 
The beating of the heart begins as soon as a connection has been made 
between this "Y" shaped tubular vessel with the vessels which have been 
formed in the vascular area. The palpitation starts at the venous, or 
caudal, end and passes to the arterial, or cephalic, end. The palpitation 
of the heart already begins before one can distinguish any definite mus- 
cular tissue which has developed from the mesoblast. 

The arterial end of the heart is known as the bulbus arteriosus. 
From this, two narrow vessels, the aortic arches, pass around the diges- 
tive tract to the dorsal side, turning caudad and becoming the dorsal 
aortae. These two dorsal aortae run along each side of the notochord 
under the mesoblastic somites and pass toward the tail unconnected 


Comparative Anatomy 

with each other, but just before reaching the tail, a large branch is given 
off. In fact, the branch is much larger than the aorta itself from v^hich 
it arises. This large branch is called the vitelline artery. The vitelline 
arteries carry the blood back to the vascular area from which it was 
brought by the vitelline veins. 

The heart we have just been describing is that of the chick. In 
cyclostomes and fishes (except the dipnoi) there is what is known as a 
branchial, or venous, heart (Fig. 446). All of the blood which enters 
such a heart is venous blood. This venous blood is pumped directly to 
the gills where it loses its carbon dioxide and takes up oxygen before 
being distributed to the various parts of the body. The important thing 
to note is that in such cases the blood only passes through the heart once 
in making its complete circuit. It is not, however, correct to consider 
the embryo of higher forms as being the same as this type of "one-heart- 
circulation," for only oxygenated blood passes through the heart in such 
embryos when it is in this stage. 

In the dipnoi and amphibia (Fig. 446), where lungs are formed to 
take up part of the work of the gills, the heart divides in an arterial, or 
systemic, and a venous, or respiratory, half. This division is caused 
by a septum, or partition, in the auricle which divides the chambers. 
It will be remembered that blood always enters through a vein and 
always enters into a sinus or an auricle of the heart. The venae cavae 



Fig. 447. 

Circulatory System 






Fig. 447. 

Comparisons of heart and connecting blood vessels in the crocodiles and birds. 
A J Dorsal view of Crocodilian heart. A.M.^ mesenteric artery; Ad and As, aortic 
arches; D.C.d. and D.C.s., right and left ducts of Cuvier; through which the venae 
cavae enter the heart; LV., pulmonary veins; L.V.h. and R.V.h., left and right 
atria; P.d. and P.s., right and left pulmonary arteries; S.d. and S.s., right and 
left subclavian arteries; Sp.i., region of the interseptal valves;, common 
carotid artery; V.c.c, Coronary vein; V.c.i., inferior vena cava. (After Rose.) 

B, Ventral View of Crocodilian Heart, anast., so-called dorsal anastomosis of 
the two roots of the aorta; anon. I., and anon.r., left and right innominate trunks; 
ao.l., left aortic arch; ao.d., dorsal aorta; ao.r,, right aortic arch; atr.l. and atr.r., 
left and right atria; coel, coeliac artery;, Botalli's Ligament; ost.atr.v., 
atrio-ventricular opening; pulm., pulmonary artery; pan., foramen of Panizzae;, aortic septum; sept. ao. pulm., aortic-pulmonary septum; sept.ventr., ven- 
tricular septum; ventr.l., and ventr.r., left and right ventricle. (After Greil.) 

C, Heart and communicating vessels of bird. (Swan.) anon, innominate 
artery; ao, aorta; ao.b., aortic arch; brach.a. and brach.v., brachial artery and 
vein; car, carotid artery cat. sup. a. and cav.siip.s., right and left superior venae 
cavae; coel, coeliac artery; cut.abd.pect., cutaneous abdominal-pectoral vein; jug.d. 
and jug.s., right and left jugular veins; Ing, lung; mnm.i., internal mammary 
artery; mes, mesenteric artery; oes, oesophagus; oes.i., inferior oesophageal artery; 
pulm.s., left pulmonary artery;, sternoclavicularis artery; subcl., subclavian 
artery; thor.inf. and thor.sup., thoracalis inferior and superior arteries; thyr, thyroid 
gland; tr, trachea;, carotid trunk; vert, vertebral artery. (After Gadow.) 

362 Comparative Anatomy 

which return the systemic blood to the heart, therefore, empty into 
the right auricle, while the pulmonary veins which carry blood from 
the lungs to. the heart enter the left auricle. As this blood which has 
returned from the lungs is now oxygenated and ready for distribution 
to the general system, it is the left side of the heart which becomes the 
arterial side. 

In the higher forms, the ventricle is also divided by a septum. The 
valves on the right side which separate the auricle from the ventricle 
are called the tricuspid valves, while those on the left side separating 
the left auricle and left ventricle are known as the mitral, or bicuspid, 

In the crocodiles, there is an opening between the two sides of the 
aortic trunk, known as the foramen Pannizae (Fig. 447, B. pan), so 
that there is really some mixture of arterial and venous blood in these 

The separation into four compartments is complete in birds and 
mammals (Figs. 445, 447, C), so that the blood must pass through the 
heart twice — once through the venous, and once through the arterial 
half — in order to make a complete circuit of the body. 

The heart is formed directly behind the mandibular artery which is 
the first aortic arch (Fig. 309), so that as other vessels grow, it is forced 
back further and further until it lies ventrad and caudad to the pharynx; 
while in the adult higher forms of mammals, it is carried back as a result 
of this unequal growth even into the thorax. (The extreme of such 
migration is seen in the girafife and the long-necked birds.) 


The ventral aorta is that larsre artery running headward from the 
heart. It extends to the mandibular artery which is another name for 
the first aortic arch (Fig. 309). The mandibular arteries, like other 
arches, pass dorsad (one on each side of the pharvnx) until they meet 
and form a pair of dorsal longitudinal tubes called the radices aortae. 
Between the first aortic arch (mandibular artery) and the heart there 
arise some six or more pairs of arches similar to those forming the man- 
dibular artery. The number of such arches depends upon the number of 
gill-clefts the animal has, for these arches develop in the septum between 
the gill-clefts. (The number of arches is greater in the myxinoids 
where the number of clefts varies ; seven or eisfht in the notidanid sharks ; 
and, as recent investigations tend to show, probably six in the embryos 
of all other vertebrates.) 

As the embryo continues to grow, the number of these arches, which 
remain or degenerate, seems to be influenced to a considerable extent 
by the various changes of the respiratory system, which the particular 

Circulatory System 


animal in question may develop. When gills develop, each aortic arch 
divides into tv\^o portions : an afferent branchial artery, v^hich carries the 
blood from the ventral aorta to the gills, and an efferent branchial artery 
w^hich carries it from the gills to the radix aortae. Both afferent and 
efferent vessels run parallel to each other for a part of their course, and 
are connected w^ith each other by numerous capillary loops running 
through the gill filaments. As the blood passes through the gills, it loses 
its carbon dioxide and takes up oxygen so that it is changed from venous 
to arterial blood. In all animals that develop an amnion, one cannot 
distinguish between afferent and efferent branchial arteries, the aortic 
arches running directly from the ventral aorta to the dorsal longitudinal 
radices aortae. 

With the possible exception of cyclostomes, no gills are ever devel- 
oped in the region of the first arch, so no afferent and efferent vessels 

Fig. 448. 

I, Aortic arches of amniotes. Compare with Figure 309. A, African Lizard 
iVaranus); B, Snake; C, Alligator; D, Bird; E, Mammal, b, basilar artery; ec, 
common carotid; cei, ce, internal and external carotids; da, dorsal aorta; p, pul- 
monary; s, subclavian. (From Kingsley after Hochstetter.) 

II, Comparison of Heart and aortic arch of crocodile and bird, a, right auricle; 
o', left auricle; ao, descending aorta; c, small connecting vessel; m, intestinal 
branches. (c and m, before they separate, form the left aortic arch) ; v. right 
ventricle; v', left ventricle; 1 and 1', carotid arteries; 2 and 2', right and left 
aortic arch; 4 and 4', pulmonary arteries. The right aortic arch, together with the 
tiny branch, c, forms the descending aorta. (After Boas.) 

364 Comparative Anatomy 

arise from the mandibular arteries. There is, however, an external and 
internal carotid artery vv^hich supply the head and brain, which come 
from each half of this first arch (Fig. 448). As various changes take 
place, however, the relation of the carotids makes them appear as though 
they arose from the first functional arch. 

In the cyclostomes and fishes, the various arches do not undergo 
much, if any, modification. Whatever modification does occur depends 
upon changes in the gills. 

In all land vertebrates and many of the fishes, the first arch on both 
sides disappears at the point where the external carotid artery begins. 

When the spiracular gill is reduced, the second pair of arches is 
partially or completely lost in the adult. 

The third pair persists. The blood for the internal carotids flows 
through these. In fishes, gymnophiona, and a few urodeles, the blood 
for the radices aortae also flows through the third pair. In all four- 
footed animals, the radix disappears between the third and fourth arches 
so that it leaves the third arch purely carotid. In such a case that part 
of the ventral aorta, which is between the third and fourth arches, carries 
the carotid blood along and hence is known as a common carotid artery 
(Fig. 448, I, B). This usually divides into a right and left branch later. 

The fourth pair form the systemic trunks in all four-footed animals 
and carry blood from the ventral to the dorsal aortae. 

The fifth has become smaller and disappears in nearly all animals 
except lizards and urodeles. In reptiles the left side of the fourth arch 
becomes separated from the rest of the ventral aorta, having its own 
trunk connected with the right side of the partially-divided ventricle, 
thus carrying a mixture of arterial and venous blood. The blood from 
the left fourth arch on the dorsal side is largely distributed to the diges- 
tive tract. The right side of the arch and the carotids are connected 
with the left side of the heart and are consequently purely arterial, the 
arch itself forming the main trunk which connects the heart with the 
dorsal aorta. In birds, the radix on the left side of the adult disappears 
caudad to where the subclavian artery begins so that this arch supplies 
blood only to the left arm. The right arch is purely aortic in character. 
In the mammals, this is entirely reversed; the right arch is subclavian 
and the left supplies the dorsal aorta and the subclavian of that side. 

The bird in its , embryonic growth (as exemplified by the chick 
embryo), turns upon its left side in about 80 per cent of all cases, and 
the right arch persists, while in mammals, the embryo usually turns upon 
its right side and the left arch persists. 

When lungs are developed, whether that be in the lung fishes or any 
of the higher forms of animals, a pair of pulmonary arteries develop 
from the sixth pair of arches on the ventral side of the pharynx. These 
arteries grow caudad into the lungs. That part of the arch dorsal to 
these newly formed pulmonary arteries becomes reduced to a small 

Circulatory System 


vessel known as the ductus arteriosus, or the duct of Botallus (Fig. 345), 
C, d, b) in some of the urodeles. In some of the higher vertebrates, one 
occasionally finds a persisting vestige of this, otherwise it entirely 

The ductus Botalli is quite important in the embryonic circulation 
of amniotes, because the greater part of the blood goes through it to 
reach the dorsal aorta during the time the allantois is the organ of respi- 
ration, while only enough blood goes through the pulmonary artery to 
nourish the lung. The duct closes with the first inspiration of air, while 
all blood passing into the last arch goes to the lung. 

In the lung fishes and amphibia, where there is but a single ventricle 
in the heart, the pulmonary arteries are connected with the ventral aorta 
just as are the other aortic arches. In higher forms, however, such as 
the amniotes, where there is either a partial or complete division of the 

Ttt nB^^Tt ^^^P.- 

Wl jL :=4sC 




Fig. 449. 

Schematic diagrams of circulatory systems in I, Petromyzon, II, Teleosts, III, 
in the higher vertebrates. I to VIII, gill arches; A, dorsal aorta; A, atrium; Ah, 
gill veins; Acd, caudal artery; All, allantoic artery; Am, omphalo-mesenteric 
artery; RA, and Aortic radices (roots of the aorta); 5". S^, two branches; 
card., anterior and posterior cardinal veins; car. ex., c, and car. int., c' external 
and internal carotid arteries; coel.mes., coeliaco-mesenteric artery; dnct.cuv., and 
D, duct of Cuvier; E, external iliac artery; HC, posterior cardinal vein; Ic, 
common iliac artery; hy.v., gill veins of the hyoid arch;, gill arteries; 
k.ven., gill veins; KL., gill clefts; ophth, ophthalmic artery; orb.nas., orbitonasal 
artery; RA, and Aortic radices (roots of the aorta); S. S^, two branches 
of the gill veins which pass into the aortic radices; Sb, Sb, and subl., subclavian 
artery; Sb^, subclavian vein; si, and sin.ven., sinus venosus; spr.k., spiracle gill; 
V, and ventr., ventricle; VC, anterior cardinal vein; Vm, omphalo-mesenteric vein. 
(From Schimkewitsch, I, after Vogt, Jung and Bridge; II, Parker; III, Wieders- 

366 Comparative Anatomy 

ventricle into tv^o portions, the conus arteriosus and the ventral aorta 
are divided so that those portions, which are derived from the sixth arch 
are connected v^ith the right side of the heart, v^hile the rest of the 
ventral aorta, with the exception already noticed in the reptiles, receives 
its blood from the left side of the heart. 

It is necessary to study the figures very carefully in order to see 
how, even in the vertebrates such as the elasmobranchs, there is a 
differentiation of the fifth and sixth arches from the rest of the series. 
It will be remembered that the fifth arch is almost completely obliterated 
in vertebrates possessing lungs, and that the sixth is completely sepa- 
rated from the rest. 

The dorsal aorta comes into existence by the fusion of two primitive 
vessels running caudad. These lie dorsal to the mesentery and run 
almost parallel to the notochord to the very end of the body. This 
fusion varies; it may extend as far forward as the aortic arch. It will 
be remembered that the portions which would normally be called the 
dorsal aortae, when these segment or when there is a division between 
the various arches, are called radices aortae. Sometimes the dorsal 
aortae extend still farther forward than the last aortic arch and involve 
the whole of the radices, so that the dorsal aorta in this case extends 
to the first arch. 

Students of comparative anatomy who are preparing for medicine, 
dentistry, and other professions, should note the fact that the names 
in human anatomy are somewhat different from those adopted in books 
on comparative anatomy. In the study of human anatomy that part of 
the ventral aorta which persists is called the ascending aorta; that part 
of the fourth arch, which continues in existence, is known as the arch 
of the aorta ; and the rest of the dorsal aorta, running downward toward 
the feet, is called the descending aorta. This, in turn, is divided into 
one portion (passing into the thorax) known as the thoracic aorta, while 
from the diaphragm downward (as it passes into the abdominal cavity) 
it is known as the abdominal aorta. The last two names are thus only 
convenient terms to show the location of the descending aorta. 


These are known as visceral (splanchnic) and somatic. As these 
terms are already familiar to the student, it is merely necessary to 
state that the visceral arteries run through the mesenteries, where the 
double layers of serosa are found, to furnish the blood supply of the 
digestive tract. Many of the blood vessels are in a primitive condition, 
though they are not metameric. Usually, especially in vertebrates, these 
smaller vessels become united into larger trunks. The principal ones 
are as follows : 

Circulatory System 367 

Coeliac Artery: 

Origin, Radix or adjacent dorsal aorta. 
Branches, Gastric, splenic, hepatic. 

Superior Mesenteric Artery (running to cephalic portion of intestines). 
Develops with the omphalomesenteric. 

Inferior Mesenteric Artery (to caudal portion of intestine). 

(Not always present. Other mesenteric arteries may also appear.) 

Coeliac Axis — is the name applied if the superior mesenteric fuses with 
the coeliac artery. 

Hypogastric Arteries: 

Originally connect dorsal aorta with subintestinal vein near anus, 
later supplying rectum. 

In animals higher than vertebrates, a urinary bladder grows from 
the rectal region, which is supplied by hypogastric branches called 
vesical arteries. 

In amniotes, where the proximal end of the allantois becomes the 
bladder, parts of the vesical arteries become allantoic arteries or umbili- 
cal arteries, because they pass through the umbilicus. With the disap- 
pearance of the allantois, these arteries degenerate, leaving only the 
rectal and vesical branches of the hypogastric trunk. 

Caudal Aorta: 

That portion of the dorsal aorta caudad to the hypogastric arteries. 


Distributed to the body wall and its derivatives. (Unlike the 
visceral arteries, the somatic arteries are arranged metamerically.) 
Intercostal Arteries (Fig. 444) : 

One pair develops between each pair of myotomes, beginning at the 
radices and the dorsal aorta. As the aortic arches disappear and change, 
the intercostals become connected close to their origin by a pair of 
vertebral arteries running through the openings in the transverse 
processes of the vertebrae. The intercostals have different names, 
depending on their location, as thoracic, lumbar, sacal, etc. 

Vertebral Arteries: 

In both man and other vertebrates, the vertebral arteries pass in a 
cephalad direction toward the ventral side of the medulla oblongata 
where the right and left arteries unite to form one trunk called the 
basilar artery. This runs straight forward underneath the brain. Two 
branches of the vertebrals extend caudad from points just before where 
the two vertebrals unite. 

368 Comparative Anatomy 

Circle of Willis : 

Just before the basilar artery reaches the hypophysis (pituitary 
body), it divides, so that one-half of the basilar passes on each side of 
the hypophysis. The internal carotid artery meets this divided basilar 
on each side, and the trunks thus formed meet near the optic chiasma, 
forming a complete arterial ring called the circle of Willis. It will be 
noticed that the brain has thus a supply of arterial blood from both 
ventral and dorsal regions, making it less likely to suffer from anything 
that might impede the circulation in any one part. 

Subclavian Arteries : 

As the limbs grow, a segmental artery, for each somite concerned 
in the appendages, extends into the member. These arteries become 
connected with each other distally, as well as with the veins of the limb, 
by a network of small vessels. The parts of these main trunks and the 
connecting network enlarge while other portions atrophy. There are 
numerous variations in the blood supply of the limbs. This explains 
the shiftings of the subclavian artery shown in Figure 448. 

The subclavian has the following names applied to different 
portions : 

Axillary — that part lying in the axilla. 

Brachial — that part lying in the upper arm. 

Radial and Ulnar — the parts lying adjacent to the bones of these 

Epigastric Arteries: 

The development of the arteries of the hind leg is somewhat com- 
plex. There is the same formation of a capillary network as with the 
fore-limb. Two of the arteries become prominent. The epigastric artery 
lies forward. It descends from the aorta to the ventral side of the 
body and forward to supply the lower portion of the myotomes. It 
becomes connected with the epigastric veins at first, although later these 
may anastomose with the hinder ends of the cutaneous arteries. When 
the hind limb grows out, the external iliac, or femoral, artery (a branch 
of the epigastric), is sent into its anterior side. As the leg increases in 
size this sometimes surpasses the parent epigastric in size, so that the 
latter appears as a mere side branch. 

Sciatic Arteries : 

The sciatic or ischiadic arteries descend into the posterior side of* 
the leg, the name changing at the angle of the knee to popliteal artery. 
Farther down, this artery divides into peroneal and anterior and pos- 
terior tibial arteries. The peroneal supplies the calf of the leg and the 
others continue into the foot. 

The arrangement of vessels here outlined is characteristic of the 

Circulatory System 369 

lower tetrapoda where the femoral artery is small. It is likewise char- 
acteristic of the embryos of mammals. In the latter, however, before 
birth, the femoral artery grows down to join the popliteal, and so 
becomes the chief supply of the limb. These trunks and the hypogastric 
do not always remain distinct. They often fuse in different ways at the 
base. Epigastric and hypogastric arteries are distinct in many reptiles 
and in birds, but in other vertebrates they fuse to form the common iliac 
artery, so called because the proximal portion of the femoral is often 
called the external, and the hypogastric the internal iliac artery. The 
sciatic, likewise, may remain distinct, or it may fuse with the others at 
the base, but then its independent portion will appear as a branch of 
the common iliac artery. 

A cutaneous artery arises from either the subclavian or the pul- 
monary artery of either side (both conditions occur in the amphibia) to 
run backward in the skin of the trunk. It may extend back and unite 
with the epigastric artery. If, as in amphibia, these arise from the 
pulmonary artery, they contain venous blood and the skin acts as a 
subsidiary respiratory organ. 

Renal Arteries: 

Renal arteries are paired and show metamerism in the primitive 
state. Details of this are given in the description of the organs they 
supply. It is well to note that metamerism is well shown in these 
arteries going to the pronephros and the mesonephros, while in the true 
kidney — the metanephros — only a single pair of renal arteries furnishes 
the blood supply. 

Genital Arteries: 

These, like the renal arteries, are paired and metameric in the primi- 
tive state and are called 
Spermatic in the male. 
Ovarian in the female. 
These are more numerous in lower animal forms than in higher. 

Omphalomesenteric Veins : 

It will be remembered that the heart is developed in the pericardial 
cavity. Caudad to the heart region, the liver begins developing and thus 
prevents the lateral plates from coming together on the ventral side as 
they did in the case of the heart. The lateral plates, however, become 
grooved, and each one forms a tube, so that there are two vessels, called 
the omphalomesenteric veins extending caudad from the heart, passing 
around the liver where they meet with the extensions of the omphalo- 
mesenteric arteries already described (Fig. 277). 

370 Comparative Anatomy 

Subintestinal Veins: 

Caudad to this connection a pair of subintestinal veins (Fig. 450) 
run toward the tail end on the ventral side of the digestive canal. These 
fuse together into a medial tube just behind the anal opening; this tube 

Fig. 450. 

Diagrams to show the development of the postcaval vein in the cat. The 
cardinal system of veins is cross-hatched, the subcardinal veins closely stippled, 
the hepatic veins are indicated by cross, vertical, and oblique hatching combined, 
the supracardinal veins by open stippling, and the renal collar by vertical hatching. 

A, early stage, showing the anterior and posterior cardinal veins, a, b, c, 
the common cardinal vein d, the subcardinal veins /, and the outgrowth e from the 
hepatic veins of the liver,_ B, next stage, showing the union of the hepatic out- 
growth e with the subcardinal veins /, to form the proximal part of the postcaval 
vein; the two subcardinals have united with each other at h. 

Cj the anterior part of the posterior cardinal vein h has separated from the 
posterior part c, c now being the renal portal vein; the postcaval vein is seen to 
be formed of the hepatic vein e, the right subcardinal /, and to be united by means 
of the two subcardinals below h with the renal portals c. 

D, the supracardinal system of veins i, represented by open stippling, has ap- 

Circulatory System 371 

extends to the end of the tail. This fused portion is known as the caudal 
vein. In the cyclostomes this connection persists. It disappears in other 

The left omphalomesenteric vein, which passes along the left side 
of the liver, continues to carry blood from the caudal or posterior part 
of the body to the heart, while the right disappears with the exception 
of the small portion between the sinus venosus and the liver. 
The Portal System: 

It will be remembered that the liver develops from a simple sac into 
a compound, tubular glandular structure. The left omphalomesenteric 
breaks up into a great mass of capillary-like tubules or sinusoids, which 
pass among the tubules of the liver and end by reconnecting at the 
cephalic end of the liver. As the liver increases in complexity so do 
these sinusoids. The left omphalomesenteric is consequently quite 
important during this period and is known as the ductus venosus 
(Arantii), (Fig. 451). A little later, however, this importance is lost 
by a part of the omphalomesenteric becoming the portal vein, which 
brings all the blood from the posterior regions of the body to the liver, 
sending it through the tiny sinusoids. The ends between the heart and 
the liver, formerly called the ductus venosus, now become the hepatic 
veins. It is in and through the hepatic veins that the collected blood 
from the liver sinusoids is sent to the heart. 

When a vein breaks up into capillaries of this kind, as in the liver 
and kidneys, and its contents are again gathered in a vein, it is called a 
portal system. That of the liver is the Hepatic portal, while that of the 
kidney the Renal portal system. 

In elasmobranchs and sauropsida, which produce eggs with large 

peared and has united anteriorly with the anterior parts of the posterior cardinals b, 
medially with the subcardinals by an anastomosis k, named the renal collar, and 
posteriorly with the renal portals c. 

E, union of the two anterior cardinals by a cross-connection p, and development 
of the renal veins from the renal collar k; the supracardinal veins have separated 
into anterior parts connected with the posterior cardinals h and posterior parts con- 
nected with the subcardinals and renal portals c. 

F, continuation of E. 

G, adult stage; the left anterior cardinal joins the right by means of the cross- 
vein p which is the left innominate vein; the common stem a, which is the right 
anterior cardinal, enters the heart by way of n, which is the right common cardinal 
vein;_ the left common cardinal vein persists as the coronary sinus o; the right 
anterior parts of the posterior cardinal vein and supracardinal form the azygos 
vein, b and i, while on the left side these are obliterated at v; the postcaval vein 
is now complete and is seen to be composed of the hepatic vein e, the right sub- 
cardinal, the anastomosis between the two subcardinals at h, the right renal collar k, 
the posterior part of the supracardinal vein i, and the posterior parts of the renal 
portals (posterior cardinals) c: the left subcardinal and posterior cardinal contribute 
to the vein of the left gonad, hence the symmetrical arrangement of the genital veins 
in mammals. 

H, composite diagram of the veins of a cat. a, anterior cardinal; b, anterior 
part of the posterior cardinal; c, posterior part of posterior cardinal or renal portal; 
d, common cardinal; e, hepatic portion of the postcaval (this is partly removed in 
Figs. D-G); f, subcardinal; g, gonad; h, union between the two subcardinals; i, 
supracardinal; ;, kidney (metanephros) ; k, renal collar or union between subcardinals 
and supracardinals; /, adrenal gland; m, vein to adrenal gland; n. base of the pre- 
caval vein or right common cardinal; o, coronary sinus or left common cardinal; 
omph.mes., omphalomesenteric artery; p, left innominate or connection between the 
two anterior cardinals; g, internal jugular; r. subclavian; s, external jugular; t, 
external iliac; u, internal iliac, (Partly from Hyman after Huntington and McClure 
in Anatomical Record, Vol. XX.) 


Comparative Anatomy 

card. p. 

omph m.d, 

- Dmph.m.s. 

yolk, the presence of a large food supply exercises a modifying influence 
on these ventral veins. A pair of large vitelline veins runs out into the 
yolk sac, over the yolk, from the junction of the omphalomesenteric and 
the subintestinal veins to play a large part in the transfer of material to 
^'"■^''- ■ the growing embryo (Fig. 284). The distal 

parts of these veins follow the margin of the 
yolk sac, forming a tube (sinus terminalis), 
into which smaller veins empty. Blood is 
brought to the yolk by the omphalomesen- 
teric arteries. These arteries are also dis- 
tributed to the yolk sac and divide up dis- 
tally into a network of capillaries which 
connect distally with the vitelline veins. 
The blood is carried by these vitelline veins 
to the liver and through the portal circula- 
tion to the heart. A similar vitelline circu- 
lation is developed in the mammals, but here 
the yolk sac contains no yolk, and so is of 
minor importance and soon lost. 

In amniotes there is an outgrowth, the 
allantois, which arises as a diverticulum 
from the hinder end of the alimentary canal. 
It increases in extent by growing down- 
ward and carrying the ventral body wall 
before it. Allantoic arteries (branches of the hypogastric arteries) 
extend into it and are connected by capillaries with umbilical veins which 
arise from the subintestinal vein behind the vitelline veins. This forms 
an allantoic circulation which is both respiratory and nutritive in char- 
acter. In the reptiles, both of the umbilical veins persist through foetal 
life, while in birds and mammals, one aborts, leaving the other as the 
efferent vessel of the allantois. With the end of foetal life (at hatching 
or at birth), both the vitelline and the allantoic circulations disappear, 
leaving only inconspicuous rudiments. 

Anterior Cardinal Veins (Superior Jugular or Jugular) : 

(The inferior jugulars are found only in fishes and salamanders, 
where they drain lateral and ventral branchial regions.) The superior 
jugular vein lies dorsal to the gill-clefts and returns blood from the 
dorsal regions of the head. 
Post-Cardinal Veins (Figs. 450, 456). 

These are very clearly related in development with the excretory 
system and lie dorsal to the coelom and dorsal to the nephridial arteries. 
Nearly all of the thoracic portion of the post-cardinal veins soon disap- 
pears in the higher forms, while a supra cardinal system develops, as 
shown in Figs. 450, 456. This supra cardinal system in turn disappears 

Fig. 451. 

Diagram showing development of 
the mammalian hepatic portal 
system. The omphalo-mesenteric 

and the umbilical veins are reduced. 
card. a. and card. p., anterior and 
posterior cardinals; d, intestine;, ductus venosus {Arantii) : /, 
liver; omph.m.d. and omph.m.s., 
right and left omphalo-mesenteric 
veins; umb.d. and umb.s., right and 
left umbilical veins. (After Hoch- 

Circulatory System 373 

with the exception of a posterior portion which takes part in the forming 
of the post-cava, and the right anterior portion which connects with the 
remnant of the post-cardinal to become the azygous vein. If the anterior 
left side persists also, this is known as the hemiazygous. 

In the lower vertebrates they retain their function of draining the 
excretory system. 

Cuvierian Ducts: 

These are formed by the meeting of the anterior cardinal and the 
post-cardinal vein on each side to form short tubes for the emptying 
of the cardinal veins into the sinus venosus. 

Subcardinal Veins: 

These are closely associated with the post-cardinals. 

As the mesenephroi in their development reach the hinder end of 
the coelom, the caudal vein loses its primitive connection with the sub- 
intestinal vein and becomes connected with a pair of vessels, the sub- 
cardinal veins, which develop in a ventral-medial position to the two 
mesonephroi. The blood from the tail now goes through the sub- 
cardinals and from them into the excretory organs, passing through a 
system of capillaries to be gathered again in the post-cardinals and by 
them to be returned to the heart. Here, then, there is another portal 
system, the first renal-portal system, which may be modified later. 

Subclavian Veins: 

One of these drains each forelimb. It originally empties into the 
post-cardinal but later may empty into the Cuvierian duct or jugular 

Common Iliac Vein: 

This drains the hind limb and empties into the epigastric (lateral 
abdominal) vein which in turn empties into the post-cardinal vein or 
duct of Cuvier. 

This is the condition in some elasmobranchs, but in the reptiles 
and amphibia, the common iliac sends part of its blood as above, and 
part through the post-cardinal of its own side, so that blood from the 
hind limbs has two routes to the heart. 

Anterior Abdominal Vein : 

The two epigastric veins in amphibia and some reptiles fuse in the- 
midline to form an anterior abdominal vein, which passes through the 
remains of the ventral mesentery (ligamentum teres) to the liver and 

In one mammal, Echidna, alone has such an anterior abdominal vein 
been found. 

The vessels of the appendages are but slightly developed in fishes. 
There is a subclavian vein which enters the Cuvierian duct, and some- 


Comparative Anatomy 

times a branchial vein which may empty into the sinus venosus. In the 
amphibia, a cutaneous magnus vein comes from the skin of the trunk, 
which may enter the subclavian. In all tetrapoda, the subclavian, after 
it leaves the limb, receives a superficial cephalic and an axillary vein. 
The latter, however, changes its name in the appendage to the brachial 
vein. The common iliac vein is formed in the limb by a union of the 
femoral and sciatic (ischiadic) veins, as well as the hypogastric (inter- 
nal iliac) vein. 

In all classes above fishes, such as dipnoi, amphibia, and amniotes, 
a new vein, the postcava (vena cava inferior) arises in part from scat- 
tered spaces and in part as a diverticulum of the sinus venosus and the 
hepatic veins. It grows backward, dorsal to the liver, until it meets and 
fuses with the right subcardinal vein, a portion of which now forms a 
new trunk to carry blood from the posterior part of the body to the 

The following changes are introduced in the embryonic renal portal 
circulation whenever a postcaval vein develops : The subcardinals no 
longer connect with the caudal vein but are connected with each other 
by transverse vessels (interrenal veins). Portions of the post-cardinals 
grow backward to connect with the caudal vein. These posterior parts 
of the post-cardinals then become the advehent veins (Fig. 452) of a 


I'yti iy% h't/er 
/'ncer<icf W'e-vi 

J^evtal aclt/ehent 

/schiapfial ^eiv, 




Hypo ^oslfit 
Cauda/ vecK 

Fig. 452. 

Diagram of Renal Portal System in A, Alligator, and B, Bird. 


Circulatory System 375 

second renal portal system. They bring blood from the tail and hind 
limbs to the excretory organs (mesonephroi). The subcardinals of both 
sides usually fuse in the middle line. The fusion is initiated by the 
appearance of the interrenal veins, which now act as revehent vessels to 
carry blood from the excretory organs to the postcava and to the anterior 
portion of the post-cardinals which have joined the anterior ends of the 
subcardinals. In mammals there is also a change in the post-cardinals 
and in the renal portal system. 

In the lung fish, Ceratodus, there are some differences from the 
foregoing changes. Here the cephalic portion of the right post-cardinal 
loses its connection with the vessels behind, and acts as a vertebral 
vein, taking the blood from the intercostal veins of that side back of the 
heart. The caudal and the subcardinals form a continuous trunk, while 
the revehent vessels form side branches. The caval portions of the post- 
cardinals grow back into the tail as paired vessels, forming no connection 
with the caudal vein. In Protopterus there is no vertebral vein, and 
the subcardinals are not fused behind, while the advehent veins are 
connected with the caudal. 

Pulmonary Veins: 

There may be various pairs of these. They carry blood from the 
lungs to the left auricle of the heart. They never empty into the sinus 


In addition to the arterial and venous divisions of the circulatory 
system, all craniates develop lymph-vessels or lymphatics. 

These consist of a network of lymph capillaries which are inter- 
woven with, but independent of, the blood-capillaries. The lymphatic 
system is not closed like the blood-vascular system, for there are not 
only definite lymph vessels, but there are large open spaces — the lymph- 
sinuses. Then, too, there are connections by little apertures, called 
stomata, between the lymphatics and the coelom. 

Lymph sinuses are found beneath the skin, as in the frog, between 
muscles, in, the mesenteries, in the walls of the alimentary tract, around 
the central nervous system, and in many other parts of the body. 

The lymph (which is practically the liquid part of the blood which 
seeps through the blood vessel walls) is gathered into these sinuses and 
then passes into more or less definite lymph vessels which, in turn, open 
into the veins (Fig. 453). 

Leukocytes are added to the plasma from the various lymphatic 
glands (Fig. 453), such as the tonsils, thymus, and spleen. 

In the lower craniates, such as the frog, lymph-hearts occur (Fig. 
347). These are muscular dilations found in the course of certain 


Comparative Anatomy 

The lymph glands (Fig. 453) are made up of a network of connec- 
tive tissue in which the lymph leukocytes (lymphocytes) are formed. 

The function of lymph glands, therefore, seems to be that of destroy- 
ing foreign bodies and to add white blood corpuscles to the general 

Am. cardinal vein 

Ant. lymph heart 

Post, cardinal 

Femoral vein 

\ . '^-' 

■— "— 

~ — 


rsf.j.. . 

^^ ^/^-^ 




-.^ tau 1.C liar, 


•""»"' ^^'"^ '••™'-"'^' 

Post. lymph heart 


Fig. 453. 


A, Diagram showing arrangement of lymphatic 
embryo. (After Sabin.) 

B, Diagram illustrating a stage in the development of a lymph gland 

20 mm. pig 

circulation. The lymph itself bathes all the cells of the body. There 
are no red blood corpuscles in lymph. 

The lymphatics of the intestine are called lacteals and perform the 
important function of absorbing fats from the ingested food. These 
lacteals combine with the lymphatic vessels from the hind limbs and 
body to form a receptacle known as the receptaculum chyli, from which 
a tube (thoracic duct) passes cephalad to open into one of the large 
veins of the precaval system by a valvular opening. The thoracic duct 
is often double. 

In mammals, the lymphatic system ramifies throughout all portions 
of the body. The lymphatic system is too delicate to be worked out by 
the ordinary laborator}^ dissection. 

AMPHIOXUS (Fig. 444) 

The blood vessels are all of one kind, but due to various homologies 
with the more complex vessels of higher animal forms, some are called 
arteries and others veins. 

The circulatory system consists of a ventral pulsating vessel with- 
out a specialized heart enlargement. This pulsating vessel pumps the 
colorless blood forward and through the branchial arches to be aerated. 
The blood then collects in paired dorsal aortae which unite back of the 
pharynx into a single dorsal aorta. Branches are sent from this dorsal 
aorta to the walls of the intestine where they break up into capillaries. 

Circulatory System 377 

The blood is collected from these capillaries into a median longitudinal 
sub-intestinal vein, through which it flows forward to pass into the 
hepatic portal vein at the origin of the liver. This portal vein breaks 
up into capillaries within the liver, and the blood is then collected in the 
hepatic vein which extends along the dorsal portion of the digestive 
gland, where it turns downward and forward to join the caudal end of 
the ventral pulsating vessel. 

The vascular system of Amphioxus, therefore, consists primarily of 

(a) a dorsal vessel represented by the paired and unpaired dorsal aortae, 

(b) a ventral vessel represented by the subintestinal vein and the ven- 
tral aorta, and (c) commissural vessels represented by the afferent and 
efferent branchial arteries and the intestinal capillaries. This is quite 
similar to the circulation in the earthworm except for two important 
differences. The blood in the ventral vessel of Amphioxus travels for- 
ward, that in the dorsal vessel backward (just the reverse of what 
occurs in the earthworm), while the ventral vessel is broken up into two 
parts, by the interposition in its course of the capillaries of the liver, so 
that all the blood from the intestine has to pass through the liver before 
reaching the ventral aorta. This passage of the intestinal blood through 
the vessels of the liver constitutes the hepatic portal system, which is 
characteristic of all vertebrates. 


The circulation in fishes corresponds quite closely in the main to 
that of the chick's embryonic circulation. It is built about the gill 
system. The blood is pumped forward from the ventral heart through the 
gills, and then, as arterial blood, it is carried backward in the dorsal 
aorta. This scheme of circulation, wherever found, is interpreted as 
primarily aquatic. 

The heart consists of four chambers : (a) sinus venosus, (b) auricle, 

(c) ventricle, and (d) conus arteriosus, through which blood passes in 
the order given. The sinus and auricle lie dorsal to the ventricle. 

In the lampreys there is no portal system. 

In the dogfish (Figs. 446, 449, 454), the circulation is laid out in 
accord with the branchial system. The blood brought to the heart by 
the venae cavae is pumped forward through a common ventral aorta 
which divides into five pairs of afferent branchial arteries, each of which 
carries blood to one set of branchiae. A corresponding efferent branchial 
vessel picks up the aerated blood from the branchiae and carries it to a 
dorsal aorta, through which it is distributed to all parts of the body, both 
anteriorly and posteriorly. The general systemic, hepatic-portal, and 
renal-portal systems return the blood to the heart along dorsal vessels 
called anterior and posterior cardinal veins. 

The fish-type of circulation is built primarily along lines laid down 
by the branchial respiration, and the heart pumps blood forward and 


Comparative Anatomy 

Fig. 454. 

A, The forepart of the body of a dogfish, dissected to show the heart and 
ventral arterial system, a.b.a., Afferent branchial arteries; au., auricle; c.a., conus 
arteriosus; ch, ceratohyal cartilage; d.C, ductus Cuvieri; g., gills; g.c, gill clefts; 
7.O., internal opening of the first gill cleft; M.c, Meckel's cartilage; mu, muscles 
from coracoid region of shoulder girdle to various parts of visceral skeleton; p.m., 
pericardium; s.v., sinus venosus; .sc, scapula; thy., thyroid gland (displaced); v., 
ventricle;, ventral aorta. 

B, The forepart of a dogfish, dissected from the ventral side, to show the 
dorsal arterial system, the olfactory organs, and certain structures in the orbits. 
The middle part of the floor of the mouth has been removed. a.b.a.. Afferent 
branchial arteries; c.c, common carotid artery; coe.a., coeliac artery;, dorsal 
aorta; e.b.a., efferent branchial arteries; e.c.,, external carotid; en., nostril; epibr., 
epibranchial _ artery;_ hm., hyomandibular cartilage; hy.a., hyoidean artery i.e., in- 
ternal carotid arteries; inf., infundibulum; M.c, Meckel's cartilage in lower jaw; 
o.i., inferior oblique muscle; o.s., superior oblique muscle; olf.o., olfactory organ; 
p.c, posterior carotid artery; sc, scapula; scL, subclavian artery; sk., skull; sp., 
spiracle;,, mandibular and maxillary branches of fifth nerve; //., 
optic nerve. (After Borradaile.) 

through the branchial arches. This involves as many pairs of branchial 
arches as there are paired functional afferent vessels carrying blood to 
the gills, and efferent vessels carrying the oxygenated blood from the 
gills to the dorsal aorta. 

AMPHIBIA (Figs. 446, 456) 

The principal changes in the amphibian circulation are concerned 
with the branchial arches. These are remodeled to become blood ves- 
sels that can function in an air-breathing animal. The branchial vessels 
of lobe-finned ganoids and of amphibia in the larval stage consist of 
four pairs, known from the region in which they develop as the third, 
fourth, fifth, and sixth. The third pair becomes the carotid arteries that 
supply the head ; the fourth becomes the systemic arches that supply 
most of the body ; the fifth disappears, and the sixth becomes mainly the 

Circulatory System 


pulmonary arches. It is of interest to note that in all lung-breathing 
fishes, the lungs are supplied from the branch of the sixth branchial arch. 
In most amphibia, a branch of the sixth arch becomes cutaneous, for the 
skin respiration is almost as important as the pulmonary. The heart is 
carried back into the trunk and consists of a sinus venosus, right and left 
auricle, ventricle, and conus arteriosus. The auricle has divided into a 
systemic half and a pulmonary half which lie in front of the ventricle. 
The single ventricle receives both arterial and venous blood, but there 
is very little mixture of the two. 

The postcava is well developed. The lateral abdominal veins (also 
called epigastric) unite to form an anterior abdominal vein. This latter 
vein permits the return of blood from the hind legs to the heart either 
through the anterior abdominal and the hepatic portal system, or the 
renal portal system and the postcava. 

REPTILIA (Figs. 447, 448, 452, 455) 

' -'--. 








Fig. 455. 

Embryonic circulation of a Snapping Turtle (Chelydra) to show the relations 
of allantois. a, right auricle; al, allantois; av, allantoic vessels; c, caudal vein; 
da, dorsal aorta; h, hypogastric artery; ;', jugular; /, liver; oa, ov, omphalo- 
mesenteric artery and vein; pc, post-cardinal; sc, subcardinal vein; uv, umbilical 
vein; w. Wolffian body; y, yolk-sac; 3-6, aortic arches. (From Kingsley after 
Agassiz and Clarke.) 

The heart is very broad laterally and consists of a sinus venosus 
(although only distinguishable in Sphenodon externally), two quite dis- 
tinct auricles (the right receiving venous blood from the body, and the 
left aerated blood from the lungs), and a ventricle always more or less 
completely divided into right and left portions. (In the crocodile the 
partition is complete.) 



Comparative Anatomy 

B. C. 

Fig. 456. 

Diagrams to show arrangement of principal veins in A, Urodele, B, Anura and 
Reptilia, C, Bird, D, Mammal. 1, Sinus venosus, gradually disappearing in the 
higher forms; 2, Ductus Cuvieri (superior vena cava); 3, Internal jugular (anterior 
cardinal sinus or vein); 4, External jugular (sub-branchial); 5, Subclavian; 6, 
Posterior cardinal, front part (venae azygos and hemiazygos of higher forms) ; 
7, Inferior vena cava; 8, Renal portal (hinder part of posterior cardinal); 9, 
Caudal; 10, Sciatic (internal iliac); 11, Femoral in A, Pelvic in B; 12, Anterior 
abdominal in A and B, coccygeomesenteric in C; 13, Femoral (external iliac) in 
B. C, and D; 14, Anastomosis of jugulars in C. (From Shipley and MacBride.) 

The sinus venosus receives the venous blood from two precaval 
(really the Cuvierian duct) and one postcaval vein. The blood passes 
through the right auricle into the right half of the ventricle, after which 
it passes through the pulmonary arteries to the lungs. From the lungs 
it returns through the pulmonary veins to the left auricle and thence to 
the left ventricle. From here it is pumped out through the paired aortic 
arches to all parts of the body. There are both a renal and an hepatic 
portal system. 

Often there is a foramen (of Panizza) connecting the right and left 
fourth aortic arches, so that blood can pass from one side to the other. 

BIRDS (Figs. 444, 446, 447, 448, 452) 

The heart is large and has two definite auricles, two ventricles, and 
no distinct sinus venosus. The right auricle receives the venous blood 
from the general body, while the left receives the aerated blood as it is 
returned from the lungs. The right aortic arch carries all of the arterial 
blood to the body-system. The renal-portal system is vestigial. 


Mammals retain the left aortic arch and lose the right, while 
birds retain the right arch and lose the left. Modern reptiles show a 
tendency to reduce the left arch. 

The valves between auricle and ventricle are tricuspid on the right 

Circulatory- System 381 

side and bicuspid (mitral) on the left. In the monotremes, however, 
both valves have three cusps. 

The pulmonary artery and aorta have three-lobed semilunar valves. 

In the monotremes, the renal portal system is better developed than 
in other mammals, although in all mammals it functions for a short 
time and disappears with the degeneration of the mesonephroi (Wolffian 

A part of the capillary system of the mesonephroi enlarges during 
the degenerative process to form a main trunk which connects the post- 
cava with the caudal portions of the post-cardinal veins. It is the post- 
cardinals that drain the tail, iliacs, and metanephroi. 

The left post-cardinal largely disappears later with the exception of 
that portion which connects with the suprarenal and gonad of the left 
side. All the blood from the posterior part of the body is, therefore, 
returned through the right post-cardinal and the postcava, whose origin 
appears to be at the union of the iliac veins. 

The postcaval vein in the turtle unites with that part of the renal 
portal system which lies caudal to the kidneys, and the renal portal 
system then passes out of existence. 

This can be understood the better if it is remembered that the renal 
portal veins are the caudal portions of the posterior cardinal veins, and 
that the subcardinal veins (particularly the right subcardinal) form the 
postcaval vein which lies between the kidneys. 

In mammals, the postcaval vein is formed principally of the distal 
ends of the posterior cardinal veins, and of the right subcardinal of the 
(vitelline) hepatic veins close to, and cephalad to, the liver, as well as 
of the hepatic veins which lie between the liver and the hind limbs. As 
the postcaval vein is made up of so many different sources, there are 
bound to be many variations in the adult state due to more or less 
persistent embryonic conditions. 

The more anterior portion of the post-cardinal veins loses its con- 
nection with the portion connecting with the excretory organs, and with 
the thoracic portion of the supracardinals, to become the azygous vein 
on the right side and the hemiazygous on the left. Either of these may 
disappear or, as in man, there may be a cross connection between these 
two veins. In such a case the anterior part of the hemiazygous is known 
as the superior intercostal vein. 

The abdominal veins are quite important in foetal life as they bring 
blood from the placenta to the embryo. 

In the higher vertebrates, including man, an innominate vein extends 
across from the carotid-subclavian trunk from one side to the other. All 
the blood is thus returned to the heart by means of the base of the right 
trunk, w^hich is now called the precava or vena cava anterior. 

The Cuvierian duct remains only as the coronary sinus. 



A S has already been noted, not only in the frog but in several of the 
/\ type-forms studied, there is an intimate connection between the 
^ ^excretory and the reproductive systems. In fact, this connection 
is so intimate that it is impossible to take up either subject without 
touching- upon the other. For this reason, it is customary to treat both 
under the head of the Urogenital System. The excretory organs, consist- 
ing of the paired kidneys or nephndic organs and their ducts, serve the 
purpose of casting out of the body the waste matter containing nitrogen, 
and occasionally other substances. 

The gonads (ovaries or testes) are the reproductive glands. To any 
and a i ui Luese, accessory structures are frequently added. The nephri- 
dic organs proper have already been quite fully described in the frog, a 
review of which is eosential to the understanding of that which follows. 

It will be remembered that the kidneys are parenchymatous glands, 
composed of a soft, more or less spongy, tissue in which there is a 
profuse quantity of blood. This great quantity of blood is sent through 
the tiny venules which anastamose with the arterial capillaries in the 
Malpighian corpuscles (Fig. 16, Vol. I). 

bome of the typical parts, which go to make up the kidneys of higher 
forms, are lacking in certain groups of animals. In the amniotes, neph- 
rost3mes are never formed, although they do occur in most ichthyopsida. 
In the pronephros, the Malpighian corpuscle is rudimentary or lacking at 
all stages while there is no differentiation of convoluted tubules and 
Henle's loop. 

Professor Kingsley's excellent account of the urogenital system is 
followed here. 

Theoretically the function of the various parts of the naphridial 
tubules is in outline as follows : In the primitive condition, the nitro- 
genous waste is elaborated in the liver, collected in the coelom and, 
together with the coelomic fluid, is passed outward through the neph- 
rostomes and the tubules which act merely as ducts. ''Higher in the 
scale the parts become more differentiated and specialized. The renal 
corpuscles form a filtering apparatus by which water is passed from the 
blood-vessels of the glomerulus into the tubules near their beginning, 
and this serves to carry out the urea, uric acid, etc., secreted by the 
glandular portions of the walls of the tubules (convoluted tubules, 
ascending limb of Henle's loop)." 

"All three ncphridia arise from the mesomeric somites or from the 
Wo ffian ridge which appears on either side of the median line where the 
mesomeres separate from the rest of the wall of the body cavity, the 

Urogenital System 383 

mesomeric cells furnishing the nephrogenous tissue from which the 
definitive organs develop." 

''Three views are held as to their relations one to another. Accord- 
ing to one they are parts of an originally continuous excretory organ 
(holonephros) which extended the length of the body cavity. This has 
become broken up into the separate parts which differ merely in time of 
development and function, with minor modifications in details. A 
second view is that they are three separate organs, while a third regards 
them as superimposed structures which occasionally overlap (birds, 
gymnophiona) and thus are not, strictly speaking, homologous but 
rather homodynamous. The first view has the most in its support, but 
for convenience the three structures are kept distinct." 

It is of considerable value to trace the successive series of these 
excretory structures in the different types of animals. It will be 
remembered that in some of the forms studied, such as the earthworm, 
there was an excretory organ in practically every segment of the animal's 
body. It will be remembered further that the so-called higher animal 
forms have practically every structure that the lower forms possess, plus 
something additional. This is well exemplified in the study of the 
nephridic organs. 

The nephridic organs of the amniotes pass through a three-fold 
development. The first excretory organ, which grows, forms the 
pronephros or head-kidney; the next succeeding is known as the 
mesonephrO'S or WolfBan body; while the last to form, which becomes 
the permanent kidney of the higher forms, is called the metanephros. 

While all three are closely related both in their development and 
their structure, there is a difference in their origin and in some of the 


As its name implies, the pronephros is the first of the excretory 
organs to appear. A review of the embryology of the excretory system 
must be had at this point. 

As the myotome is being formed from the epimere, the dorsal end of 
each mesomere closes. This forms a sac which opens into the coelom. 
Each of these is called a nephrotome and lies a little behind the head. It 
is from these nephrotomes that the pronephros is formed. The number 
of pronephridic organs varies from one in the teleosts to a dozen or more 
in the caecilians. The usual number, however, in the higher forms is 
two. From the somatic walls of these nephrotomes there is an out- 
growth toward the ectoderm. This forms slender pronephric tubules as 
in the amphibia, or solid cords which later have a lumen form within 
them as in elasmobranchs and amniotes. They thus all become tubules ; 
the proximal ends of each communicates with the metacoele by way of 
the cavity in the nephrotome. The opening to the metacoele is called a 
nephrostome and, as already noted, there will be as many tubules and 

384 Comparative Anatomy 

nephrostomes as there are somites. The distal ends of the nephrotomes 
grow outward until they reach just below the ectoderm when they bend 
toward the caudal end of the body. Here the more cephalic tubules fuse 
with those behind and it is at this meeting place of the tube that the 
pronephric duct, sometimes called the archinephric duct, grows back- 
ward immediately beneath the ectoderm. This backward growth con- 
tinues until the caudal end of the metacoele has been reached. It is here 
that the pronephric duct fuses with the caudal end of the digestive tract 
and empties into the cloaca, as in the frog, where it meets with the 
ectoderm close to the anal opening. In either case an opening then 
breaks through so that the contents of the duct can be expelled. 

The question is often asked as to whether the ducts thus formed 
are of mesothelial origin or whether the ectoderm contributes a share. 
From present evidence it is assumed that the ectoderm has no share 
in their formation. 

The pronephros functions for a time in embryos of some of the 
lower vertebrates, while in higher forms only a part remains as the 
oviducts and the ostium tubae abdominale of the female. 

During the functional embryonic period the pronephros carries 
nitrogenous waste from the body-cavity. Its filtering apparatus is made 
up either of a separate glomerulus to each tubule or a group of glomeruli 
from the separate somites to form a glomus. 

Neither the glomeruli nor the glomus project into Bowman's cap- 
sule, but, lie directly above the dorsal wall of the coelom between 
mesentery and nephrostomes, pushing the epithelium before them. 

Later, both glomeruli (or glomus) and nephrostomes may become 
enclosed in a cavity which has become cut oiT from the coelom, appear- 
ing quite like a renal capsule, although the renal capsule is different in 

The nitrogenous fluid passes into the metacoele from where it is 
drawn by the cilia of the nephrostomes to pass along through the 

Short segmental arteries from the dorsal aorta bring the blood to 
the glomeruli or glomus. After this blood has flown through the capil- 
laries, it passes through the post-cardinal veins to the heart. The post- 
cardinals develop backward just as rapidly as the tubules of the 
pronephros grow in that direction. 

In all vertebrate adults, with the possible exception of Bdellostoma 
(Fig. 366), the pronephros has been replaced by the mesonephros and 
later still in the amniotes by the metanephros. In the cyclostomes and 
a few teleosts, the pronephros, however, persists. 

The mesonephros, also called the Wolffian body, is formed by a 
series of mesonephric tubules which are developed after the pronephros 
and its ducts are completely formed. The mesonephric tubules grow 

Urogenital System 385 

out from the nephrotomes behind those which form the pronephros. The 
tubules extend toward each side of the animal until they meet and fuse 
with the pronephric duct. This duct is then the excretory canal for the 
mesonephros. The points of origin of the mesonephric tubules vary in 
different animals. Some lie dorsal to the pronephric tubules, while two 
arise from the same nephrotome one above the other. In fish and 
amphibians, the nephrostome consists of the opening of the nephrotome 
into the metacoele. As this opening, however, is closed in the amniotes 
even before the tubules are formed, there are no nephrostomes, and con- 
sequently there is no connection between tubules and the peritoneal 

In nearly all the higher forms (in some rodents this is not true) 
segmental arteries from the aorta grow out to the splanchnic wall of 
each nephrotome to form a network of capillaries. These growths take 
place at a level somewhat higher than the pronephric glomeruli. 

The capillaries form glomeruli which press against the wall of the 
nephrotome. The rest of the nephrotome closes around the pressed-in 
portion to form a Bowman's capsule. The capsule, together with the 
enclosed capillaries, is a Malpighian body. 

In most ichthyopsida, the nephrostome connects the Malpighian 
body with the metacoele on one side, while the mesonephric tubule 
connects with it on the other. 

The mesonephros is metameric at first. It extends over many more 
somites than the pronephros, reaching nearly to the posterior limits of 
the metacoele. 

As the embryo develops, the number of tubules increases (in all 
vertebrates except the mxyinoids) by budding. The tubules then unite 
with those first formed so as to form collecting tubules from their distal 
portions. A separate Malpighian body is formed for each of the sec- 
ondary tubules. All the tubules elongate, become convoluted, and the 
mesonephros ceases to be metameric. 

Changes in circulation are also taking place. The veins which carry 
blood from the renal corpuscles extend into the region of the tubules. 
Each vein breaks up into a second system of capillaries which envelop 
the tubules before returning the blood through the post-cardinal vein. 
It is the subcardinal vein which brings the blood from the posterior 
body-region to^ the Wolffian body, from w^hence it is returned to the 
heart through the post-cardinals. 


In those animals, seemingly more primitive, such as the elasmo- 
branchs and in some of the amphibia, the pronephric duct divides longi- 
tudinally from its most caudal end forward almost to the cephalic end of 
the Wolffian body. This occurs at the time the mesonephros develops. 
There are thus two ducts formed one of which, called Wolffian, or Ley- 
dig's, duct, remains connected with the tubules of the mesonephros and 

386 Comparative Anatomy 

forms its excretory canal, while the other, called the MuUerian duct, is 
also quite closely related to the pronephros, but forms the oviduct in the 
female. In the amniotes, the pronephric duct does not divide but 
becomes the Wolffian duct, while the oviduct arises in another manner. 
(See page 390.) This same condition is found in many of the amphibians 
and in all of the teleosts. 


While the mesonephros functions in all vertebrate embryos and 
throughout the entire life of fish and amphibians, as well as a short time 
after birth in the lizards and opposum, this organ becomes replaced in 
the adult of all amniotes by the two metanephridic organs which form 
the true kidneys. Each of this pair of kidneys takes its origin directly 
behind the mesonephros of the same side, while from the caudal end of 
the Wolffian duct, close to its entrance into the cloaca, a tube, the ureter, 
grows forward, parallel to the parent duct, into the tissue caudal and 
dorsal to the mesonephros. It is supposed that this is more or less 
metameric although all trace of such metamerism has disappeared, the 
kidneys not being segmented at any stage of their development. The 
cephalic end of the ureter has a varying number of branches, whose tips 
expand to form what is called a primary renal vesicle. Around each 
primary vesicle, a group of cells develops, the aggregate of which grows 
into an "S" shaped tubule, one end of which connects with the primary 
renal vesicle, the other developing into a glomerulus. There are no neph- 
rostomes. Still later these tubules multiply extensively and the blood 
capillary system of the glomerulus increases also. 


This reservoir for urine, also called a urocyst, forms toward the 
caudal end of the excretory ducts. There are three kinds of urocysts : 

1. A bladder arising by the fusion of the caudal ends of the Wolffian 
ducts plus a portion of the digestive tract. This is the cloaca type. The 
Wolffian ducts in such cases empty into the cloaca, the cloaca then 
opening to the exterior. 

2. The usual urinary bladder formed by a diverticulum from the 
dorsal wall of the cloaca cephalad to the openings of the Wolffian ducts. 
It is supposed, however, that this form may be homologous with the 
rectal gland of the elasmobranchs. 

3. The allantoic bladder occurring in all higher forms as a ventral 
diverticulum from the cloaca. The entire outgrowth forms the bladder 
in amphibia, while in the amniotes only the proximal portion becomes 
the bladder. The distal portion is used in the embryo as a respiratory 
organ, the allantois. The allantois is quite extensive and forms a part 
of the placenta in mammals. It is either absorbed or lost at or before 
the time of birth. 

In the higher forms in which a bladder is present, the ureters open 

Urogenital vSystem 387 

directly into it, the urine being conveyed to the exterior through the 
single tube, the urethra; in amphibia the urine must first pass through 
the cloaca before entering the bladder, as the Wolffian ducts do not 
directly enter the urocyst. In many birds and reptiles there is no urinary 
bladder at all, although these have an allantois during their embryonic 

The nephridic tubules are quite like those studied in the earthvi^orm. 
The nephrostomes open into the coelom w^hile convoluted tubules 
envelop a network of capillaries. In the earthworm each tubule opens 
separately to the exterior in the somite behind the one in which the 
nephrostome lies ; but, in vertebrates the whole series of tubules empty 
into a common duct. Much work is still needed to explain the nephridic 
system satisfactorily. 


A detailed study of the embryological beginnings and development 
of the reproductive organs has already been covered in that part of 
this book devoted to embryology. After this has been reviev/ed it will 
be understood how the germ plasm from which the gonads develop is 
set aside very early in the growing embryo. The gonads are not seg- 
mented, notwithstanding the fact that earlier writers have taken another 

These sexual organs in their growth, push a layer of peritoneum 
before them just as do the other outgrowths in the body. Such peri- 
toneum covering the male gonad, which serves as a support for the 
testes, is called a mesorchium, while that supporting the ovaries is 
known as a mesovarium. In all the higher forms gonads are paired. 
In many fishes and birds they are unpaired, due to a fusion of two or to 
a degeneration of one. 

We have seen in our embryological study how the gametes are 
formed in the female and lie within a Graafian follicle which, after rup- 
ture, leaves a scar in the form of a corpus luteum, while in the male, 
instead of the primordial ova and the epithelial cells becoming separate 
follicles, they develop into a cord which later on has a lumen open 
through it to become the seminiferous tubule. Both epithelial cells and 
primordial spermatagonia may be found in the walls of this tubule. A 
third type, known as Sertoli's cell, is also found here. These latter are 
called nutritive or nurse cells for the developing spermatozoa. Just what 
function these cells have aside from this supposed nursing, is unknown. 
The testes remain in the position where they first appear in most verte- 
brates, but in nearly all the mammals they descend to assume a position 
outside the body cavity. They are enclosed in a special pouch called 
the scrotum. ^^^ REPRODUCTIVE DUCTS 

As fertilization is necessary in at least all the higher forms of 
animals, there must be some method by which the sperm or the eggs, 


Comparative Anatomy 

as well as the young in viviparous animals, may be carried to the outside 
of the body. The sperm-ducts of the mammal are known as vasa defer- 
entia. (Fig. 457.) The tgg ducts of the female are called oviducts 
(Fallopian tubes). The vasa deferentia are usually the Wolffian ducts, 
but in the female the oviducts may be either the Miillerian ducts, or 
specially developed tubes, or even merely abdominal pores. In prac- 
tically all the forms we are studying, the Wolffian ducts serve as the 
outlet for the sperm. 

At the same time that the tubules, which are to carry the 
sperm, are developing, there is an outgrowth of cells from the Bow- 
man's capsules at the cephalic end of the mesonephros to form 
medullary cords. These latter continue their growth into the genital 
ridge until they connect with the seminiferous tubules. All of these 
acquire a lumen and both together form a continuous transverse 
tubule, known as the vasa efferens. (Fig. 458.) This continuous tube 
leads from the genital cells to the Malpighian corpuscles and thence 
by the mesonephric tubules to the Wolffian duct. The vasa efferentia 
become connected by a longitudinal canal before actually entering the 
Wolffian body. There is also usually a second longitudinal canal 
which connects them in the body of the testes. The connection of testis 

Fig. 457. 
■' 't/^fifagrams of urogenital systems of female fishes. A, Afri- 
can lungfish Protopterus; B, African ganoid Polypterus; Ameri- 
can ga^fl"*^«jjia; B4\0ie garpike Lepidosteus; E, most teleosts; 
F, trou? arid salmon. ap, abdominal pore; ch, cloacal bladder; 
c/, ,|cloaca; /^i funijel of oviduct; gp, genital pore or papilla; m, 
rriesonephrbsr o; ovary; od, oviduct; r, rectum; s, urogenital sinus; 
up, urinary pore, (papilla); ugp, urogenital pore (papilla); w, 
WolAiari ducts. (From Kingsley after Goodrich.) 

Urogenital System 


Fig. 457. 

II. Diagrammatic representation of the ' modifications of the 
urogenital ducts. 1, 2, male and female Newt. 3, a tubule of 
the kidney. 4, Male Frog. 5, Male Toad. 6, Male Bombinator 
(European Frog). 7, Male Discoglossus (Fire-bellied toad). 8, 
Male Alytes (obstetrical toad), d, artery entering and producing a 
coil in the Malpighian body, M; B, Bidder's organ; ef.s.c, 
efferent segmental canal; F.B, fat-body; gl, glomerulus; K, kidney; 
l.c.c, longitudinal collecting tubule; M, Malpighian body; Md, 
Miillerian duct; A'^, nephrostome; O, ovary; ov, oviduct; s.d., seg- 
mental duct; T, testis; Ur, ureter; V.d., vas deferens; V.s., 
seminal vesicle. (After Gadow.) 

and Wolffian body, while usually taking place at the cephalic end of 
the mesonephros, may, as in some of the lung fishes, take place at the 
caudal end. 

At about this time the glomeruli of the tubules degenerate. This 
means that the part of the mesonephros in which these glomeruli degen- 
erate is no longer excretory, but has become a part of the reproductive 

390 Comparative Anatomy 

system. It will be noted, then, that sperm can pass throughout the 
lumen of a tube the entire distance from their origin to their exit from 
the body. 

The cephalic end of the Wolffian duct becomes purely reproductive 
in the male, it being considerably coiled to form the epididymis. (Fig. 
458.) In the amniotes, where the hinder portion of the mesonphros is 
supplanted by the true kidney (metanephros), the whole Wolffian duct 
is a sperm duct (vas deferens) in the male, while in the female it largely 
or completely degenerates. In the amphibia and elasmobranchs the 
hinder end of the duct is both reproductive and excretory in the male. 
In the female it is purely excretory. 

"In the ichthyopsida, other than elasmobranchs and amphibia, the 
sperm is carried to the exterior in other ways, and there is no connection 
of the testes with the excretory organs. In the cyclostomes the sperm 
escapes from the testes into the coelom and then is passed to the exterior 
by way of the abdominal pores which in the lampreys open into a cavity 
(sinus urogenitalis, Fig. 457) which also receives the hinder ends of the 
Wolffian ducts. In the myxinoids the pores are united, and they open to 
the exterior behind the anus and between it and the urinary openings." 


As already stated, in many forms, the Miillerian duct is the direct 
result of the splitting in two of the pronephric duct which then serves as 
the oviduct. At its separation from the Wolffian duct the Miillerian 
duct opens into the coelom by means of the pronephric tubules and their 
nephrostomes. These then flow together and form a larger opening, 
called the ostium tubae abdominale, on each side. (Fig. 458.) In the 
elasmobranchs the ostia are usually united ventral to the liver. The 
eggs which are thrown out of the ovaries into the coelom are picked up 
by the somewhat trumpet shaped extensions around the ostia and carried 
into the oviduct. In some forms the pronephric tubules and nephro- 
stomes take part in the formation of the ostium tubae and the beginning 
of the oviduct; however, as in all the higher forms, the remainder of 
the oviduct arises by the formation of a groove of the peritoneal mem- 
brane close beside the Wolffian duct. This becomes rolled into a tube 
to form the Miillerian duct. In the amniotes the anterior end of the 
groove does not close, but remains open as the ostium tubae. (Fig. 458.) 

It is difficult to trace the successive stages from the most primitive 
to the most highly developed types of oviducts. Some writers regard the 
condition of the oviduct in the elasmobranchs as the most primitive. 
Some contend that we have here a homologous condition — a condition 
resulting from similar primitive structures; others that it is rather analo- 
gous and an example of convergent evolution in that these organs, 
having been used for similar functions, have come to appear somewhat 
alike structurally. 

Urogenital System 


It can be seen how difficult valid comparisons are when we have 
such varying- conditions in the lower, but nevertheless supposely related 
forms, as this : In the cyclostomes the eggs are thrown from the ovaries 

vasa efferentia 

epididymis (vestige 
•f mesonephros) 


Fig. 458. 

Diagrams to illustrate the urogenital system of male and female anamniotes and 
amniotes. A, male elasmobranch or amphibian; the mesonepheros is differentiated 
into anterior genital and posterio-excretory portions; the genital part is connected 
with the testis by means of the vasa efferentia, which are outgrowths from the 
mesonepheros, the mesonepheric or Wolffian duct serves as both genital and ex- 
cretory duct; the oviduct or Miillerian duct is vestigial. B, female elasmobranch or 
amphibian; the ovary is not connected with the mesonephros; the mesonephros 
and mesonephric duct serve only excretory functions; the oviduct is well developed 
and opens into the coelom by the ostium near the ovary. C, male reptile, bird, or 
mammal. The excretory part of the mesonephros has disappeared but the genital 
part persists as the epididymis (in part) which is connected as in anamniotes with 
the testis by means of the vasa efferentia; the Wolffian duct is purely genital and 
is renamed the vas deferens; the excretory function is served by the metanephroi 
and ureters. D, female reptile, bird or mammal; the mesonephros and Wolffian 
duct have entirely vanished; the condition of the ovary and oviduct is the same 
as in anamniotes; the excretory function is served by the metanephroi and ureters 
exactly as in the male. (From Hyman's modification of Wilder.) 

392 Comparative Anatomy 

directly into the coelom, being passed out through abdominal pores; in 
the teleosts alone there are several conditions, the ovaries of some are 
simple and composed of solid bands or are sac-like, having an internal 
lumen. In the simple forms the eggs pass into the coelom and thence to 
the exterior by abdominal pores or by oviducts of varying lengths. We 
do not know whether these ducts are true Miillerian ducts or whether 
they are new formations. 

The sacular condition of the ovaries may come about by the free 
edge of the ovary bending laterally and fusing with the wall of the 
coelom. This forms a cavity, called the parovarial canal, closed in front. 
Or there may be a groove in the covering epithelium forming on the 
surface of the ovary. In this case, as it closes and sinks inward, it forms 
what is called an entovarial canal. In either case the canal may extend 
to the most caudal end of the body cavity and form an oviduct in this 
manner, or the oviduct may be formed from both kinds of canals, one 
in front, the other behind. 

''From this it would appear that the ovary originally extended back 
to the hinder end of the coelom (as it does in Cyclopterus) or that the 
par- or entovarial canal had united with a Miillerian duct which has 
otherwise been entirely lost. The oviducts thus formed usually unite 
before opening to the exterior, either directly or via a urogenital sinus." 

It will be remembered that there are shell glands (likewise called 
nidamental glands) in those animals which are oviparous, although these 
may appear in viviparous forms also; they are but slightly developed in 
these latter instances. It is interesting to note that in some species of 
elasmobranchs the eggs are larger than those of an ostrich. In this same 
type of animal the caudal or inner side of the pelvic fin is specialized 
for a copulatory organ as fertilization is internal in the elasmobranch. 
In the amphibia, there are many interesting accessory reproductive rela- 
tions, as mentioned in the chapter on classification of vertebrates. The 
caecilians and Amphiuma lay their eggs in long strings in the soil and 
the female incubates them, although the male often takes charge of the 
eggs. In Pipa, each egg undergoes development in a pit in the skin of 
the back of the female, and in Nototrema and Opisthodelphys (South 
American tree-toads), there is a large pocket in the skin of the back, 
opening near the coccyx, where the eggs are carried until partially 
(Nototrema) or entirely developed. Salamandra maculosa and S. atra 
bring forth living young, the former possessing gills at birth, the latter 
in the adult form. 

In the higher forms of vertebrates there is a definite single copula- 
tory organ. Among the sauropsida Sphenodon alone lacks all copulatory 
organs, while in most birds they are incomplete. The males of croco- 
diles, turtles, ostriches, ducks, geese, and swans are among the very 
few that have a definite structure homologous to that of mammals for 

Urogenital System 


this purpose. In snakes and lizards, several sacs are developed from the 
caudal wall of the transverse anus. They resemble appendages in the 
embryo and form real copulatory organs called hemipenes. They are 
present in both sexes though very small in the female. As growth con- 
tinues, retractor muscles are developed which draw the organs back into 
pockets where they are retained at all times except when used for copu- 
lation. The simplest form of the copulatory organ is produced by a 
thickening of the ventral wall of the cloaca. There is a longitudinal 
groove, formed in the upper surface of this, through which the sperm 
may pass. It may be divided into right and left halves, the tip of which 
forms the glans penis. The homologous structure is the clitoris which 
forms in the female though all parts but the glans are lacking. 

In the mammals, while there are two pronephric tubules outlined in 
the embryo, they never are functional and the pronephros degenerates. 
The mesonephros, however, is definitely used during foetal life, and in 
the marsupials and monotremes it even functions sometimes after birth. 
However, in all forms of mammals it disappears in time, with the excep- 
tion of the efferent ductules of the testes and a few remnants in both 
sexes. The metanephros, which becomes the permanent kidney, has 
several lobes in the early stages. A definite lobe is formed for each 
end branch of the ureter so that each lobule has it own duct. This 
condition is retained in "adult elephants, some ungulates, carnivores and 

primates, and especially in the aquatic 
species (whales, seals), the lobules being 
most numerous (200+) in some whales. In 
all other species the ducts fuse and the 
lobules unite later into a compact mass 
lying in the lumbar region near the last rib." 
These lobules are the cause of the cortex 
and medulla of the kidneys forming two 
series of interlocking pyramids. (Fig. 459). 
In the early embryonic stages the 
gonads lie cephalad to the kidneys. 

The ovaries are usually equally de- 
veloped in the mammals except in the 
monotremes ; here the left is the larger. "It 
is of interest that eggs — one in the Echidna, 
two in Ornithorhynchus — have been found 
only in the left oviduct." The ovaries, 
unlike the testes, always remain in the 
body, and in the monotremes retain their 
early position. "They are supported by the 
mesovaria which are attached to the median 
side of the double fold of the peritoneum 
which supports the mesonephros. When 

Fig. 459. 

Longitudinal section through 
kidney. 1, cortex, 1', medullary 
rays; 1", labyrinth; 2, medulla; 2', 
papillary portion of medulla; 2", 
boundary of medulla; 3, transverse 
section of tubules in the boundary 
layer; 4, fat of renal sinus; 5, 
artery; transverse medullary rays; 
A, branch of renal artery; C, 
renal calyx; U, ureter. The pyra- 
mids are located between the fat 
portions and form the papillae. 
(From Hill after Tyson and 


Comparative Anatomy 

the Wolffian body degenerates, the fold becomes the broad ligament 
while another fold continues down the Miillerian duct as the ligament 
of the ovary. In some mammals the ovaries have, in addition, a special 
fold of the peritoneum, which in the rats and mice encloses the ovary 
and the ostium tubae connected with its opening." 

"The testes are relatively small and are shaped much like the ovaries 
and at first they are at about the same level. The outer surface is smooth, 
a fibrous envelope, the tunica albuginea, having developed around 
them, which sends trabeculae inward, dividing the seminiferous tubules 
into lobules. Except in the monotremes, the testes descend farther 
into the pelvic cavity, remaining permanently in the pelvis in many in- 
sectivores, some edentates, elephants, whales and Hyrax. In other 
groups they pass outside the pelvic cavity to be enclosed in a special sac, 
the scrotum. The testes are supported by a cord, the gubernaculum, 
the homologue of both ligaments of the ovary. 

*'The change in position of ovary and testis is accomplished in part 
by the unequal growth of body wall and the supporting ligaments. In 
the case of the male this descent of the testes is complicated. (Fig. 460.) 
In outline it is as follows : By the unequal growth of gubernaculum and 
body wall, the testes are drawn down into the scrotum which is a pro- 
truding part of the body wall into which a part of the coelom extends. 
This wall is formed in part from the genital folds which surround the 
genital eminence. It lies in front of the penis in the marsupials, behind 
it in all placental mammals. When the canal connecting the cavity of 
the scrotum (bursa inguinalis) remains open as it does in marsupials, 
bats, rodents, insectivores, etc., the descent is temporary, the testes 
being withdrawn into the peritoneal cavity at the close of the breeding 
season by the cremaster muscle, developed from the transverse abdomi- 
nal muscle. In other mammals the descent is permanent, though some- 
times it does not occur until the time of sexual maturity." 

In the monotremes, the 
Miillerian duct is divided into 
a cephalic portion, known as 
the Fallopian tube, and a cau- 
dal portion, the uterus, al- 
though the line separating 
these two is not very definite. 
The broad trumpet-shaped end 
of the Fallopian tube connects 
with the coelom, while the tube 
itself secretes the albuminous 
covering of the eggs. The uterus is more muscular than the Fallopian 
tube, and it is here that the horny shell is formed. The uterus then 
opens directly into the urogenital sinus to connect the cloaca with the 

Fig. 460. 

Descent of the testis ac, abdominal cavity; g, 
gubernaculum; pv ; processus vaginalis; t, testis; s, 
scrotum; tv, tunica vaginalis; x, rudiment of processus 

Urogenital System 


In other forms of mammals, the end of the Miillerian duct between 
the uterus and the urogenital sinus forms a vagina. In marsupials there 
are two vaginae and sometimes three. 

When the two caudal ends of the Miillerian ducts fuse as in many 
placental animals, such as in rodents, two uteri are formed, each with 
a separate opening into the vagina. (Fig. 461.) In the carnivores and 
ruminants where the fusion is carried still farther back, forming in 
reality two uteri with only one opening, it is called a uterus bipartibus; 
where it is carried still farther, forming two horns, it is called uterus 
bicornuus. For the uterus simplex the fusion is entirely complete, as 
in all primates, the two Fallopian tubes alone remaining as evidence of 
its bilateral formation. 

In the female, the Wolffian duct and the mesonephros are largely 

Five varying uteri. A Monotreme; B, Marsupials; C, duplex uterus; D, 
bicornuate uterus, and E, Simple uterus, ost.abd., abdominal opening (ostium) 
into oviduct; ovid., oviduct; s.u.g., urogenital sinus; ut, uterus; vag., vagina; 
ves., urinary bladder. Such uteri as A and B open into the urogenital sinus, 
while C, D, and E, open into the vagina. (After M. Weber.) 

lost in the adult; the mesonephros forms a small collection of tubules 
near the anterior end of the ovary which is known as the parovarium. 
The Miillerian duct in the male is also largely lost, the lower portion 
sometimes persisting as a small blind tubule imbedded in the prostate 
gland and known as the uterus masculinus. (Fig. 462.) 

Between the tubules in the testes there are small aggregates of cells 
known as interstitial cells, which are glands of internal secretion. In 
man, their products, which pass into the blood, apparently cause the 
assumption of the secondary male characters — growth of hair on the 
face, change of voice, etc. — at the time of puberty. There would also 
seem to be some analogous structure in the ovary governing the devel- 
opment of female characteristics and controlling some of the features 
of menstruation. 

There are also a number of accessory glands (Fig. 462) connected 
with the genital ducts, usually better developed in the male than in the 
female. The more prominent ones are : the seminal vesicles (present in 
some rodents, bats, insectivores and in ungulates and primates), a pair 
of tubular or saccular glands opening into the vasa deferentia just before 
these enter into the urogenital canal ; the prostate glands (occurring 


Comparative Anatomy 

in all placental mammals with the exception of edentates and whales), 
connected with the urogenital canal; and farther along, the canal 
Cowper's glands are found. These occur in almost all mammals as 
scattered bodies or aggregated into larger masses surrounded by smooth 

Considerable uncertainty exists as to the exact functions of any of 
these glands. The removal of the prostate and the seminal vesicle in 
rats prevents fertilization, while the secretion of the seminal vesicles 
increases the activity of the spermatozoa. It seems probable that they 
are of great importance in connection with fertilization. It has also 
been shown that in some instances the coagulation of the secretion of 
these glands closes the vagina after copulation and thus prevents the 
exit of the sperm. 

In the monotremes, the cloaca serves as a general gathering place 
for both the products of the urogenital sinus and the excreted matter 

c.cav ; 


P-9' 'r,n \ Gp. 


Fig. 462. 

A B The reproductive organs of the rabbit. A, male; B, female. In each 
case the dissection is made from the left side, the animal lying on its back, bl., 
Bladder; c.cav., corpus cavernosum;, corpus cavernosum of the clitoris; 
Cp., Cowper's gland; epd., cauda epididymis; epd'., caput epidiymis; F.t., 
Fallopian tube;_ j.o., fimbriated opening of the same; ov., ovary; p.g., perineal 
gland; pn., penis; pr., prostate; r.g., rectal gland; rni., rectum; sc.s., scrotal sac; 
sp.c, spermatic cord (cut short) ; ^:ym., symphysis pubis; t., testis; ur., ureter; 
ut., uterus; ut.m., uterus masculinus; uth., urethra; v.d., vas deferens; vag., 
vagina; vest., vestibule. 

C, male, and D, female reproductive organs of dogfish, cl.. Cloaca; /./., "falci- 
form", ligament; i.o.d., rudiment of the internal opening of the oviducts; l.t., left 
testis; msn., mesonephros; mso., mesorchivtm; mtn., metanephros; od., oviduct; 
oes., oesophagus; ov., ovary; r.t., right testis; rm., rectum; sh., shell gland; sp.s., 
sperm sac; u.g.p. and v. p., urinogenital papilla; u.g.s. and ur.s., urinogenital sinus; 
ur., ureter; ur'., ducts of metanephros; v.eff., vasa efferentia; ves.sem., vesicula 
seminalis; W.d., Wolffian duct or vas deferens. 

Urogenital System 


from the digestive canal and kidneys. This cloaca has only a single 
opening to the exterior and it is from this fact that the name monotreme 
has been taken. In all other mammals there is a definite and complete 
separation of the faecal and urogenital matter. This separation is 
brought about by a horizontal partition dividing the cloaca into a dorsal 
rectum and a ventral urogenital portion. This space between rectum 
and urogenital portion is called the perineum. 

Fig. 462. 

E, The urogenital organs of a female pigeon. K,_ kidney (metanephros) with 
three lobes; u., ureter; ci., cloaca; ov., ovary; od., oviduct; f.t., funnel at end of 
oviduct; r.r.od., rudimentary right oviduct. 

F, The urogenital organs of a male pigeon, T., testes; V., base of inferior 
vena cava; S.R., suprarenal glands; K., kidneys with three lobes (1, 2, 3); »., 
ureter; v.d., vas deferens; v.s., seminal vesicle; cL, cloaca. (A, B, C, D, from 
Borradaile; E, F, from Thomson.) 


In both sexes of mammals, the same anlagen of the external genitalia 
are found as already noted in the study of embryology. These consist 
of a genital prominence which is formed from the ventral or anterior 
wall of the cloaca. This then protrudes from the opening and, when 
the perineum is formed, two thickenings appear on each side, a medial 
genital fold and a larger and lateral ridge, which extends back nearly. 
to the level of the anus. The genital prominence never develops much 
farther in the female, while the folds and ridges become the labia minora 
and majora. In the male, however, a groove is formed on the primitively 
dorsal surface of the prominence which continues into the cloaca. Then 
the folds grow together behind the prominence, closing the groove so as 
to form a tube, the urethra, and the prominence becomes the glans penis. 


Comparative Anatomy 




A similar growth of the genital ridges toward the median line results in 
the formation of the outer wall of the scrotum. 

While internal fertilization 
takes place in most of the higher 
forms of animals, there are many 
vertebrates, such as the cyclostomes, 
most fishes with the exception of the 
elasmobranchs, and many amphibia 
in which fertilization does not take 
place until after the eggs have 
passed from the body of the female. 
The organs by which sperm is 
passed to the female are formed in 
many ways and are not considered 
homologous in the different forms. 

As we already know from the 
study of the earthworm, there are 
animals possessing both ovaries and 
testes. Such animals are commonly 
termed hermaphrodites. True her- 
maphrodites must have both ovaries 
and testes functional. (Fig. 463.) 
It is interesting to note that, while there are occasional hermaphro- 
dites among the lampreys, this is a rather common occurrence in the 
myxinoids. In these the cephalic end of the gonad is male, while the 
caudal end is female. However, usually, only one or the other of these 
functions, so that the animal is either predominantly male or female. 
Hermaphroditism has been found among the frogs, while in toads there 
is often a "Bidder's organ" lying directly in front of the gonads of the 
male but containing immature ova. (Fig. 457.) Cases of hermaphrodit- 
ism, although possible, are seldom found in mammals, the so-called cases 
being merely arrested growth in the male, preventing the two portions 
of the scrotum from joining in the mid-line, or an hypertrophy of the 
clitoris in the female. 


Fig. 463. 

Hermaphrodite Frog, f.k., fat-bodies; n.l. 
and n.r., left and right mesonephroi; ovid.l. 
and ovid.r., left and right oviducts; test. I. and 
test.r., left and right testes. (After Mitro- 

Closely associated with the nephridial structures lie two small duct- 
less glands, one connected with each renal organ in the higher forms. 
In the lower vertebrates each one of these is in turn composed of two 
structures. In the amphibia and amniotes, one portion, called the supra- 
renal, forms the medulla, while the interrenal forms the cortex of the 
mammalian adrenals. (Fig. 351.) The suprarenal portion is always 
connected with the sympathetic nerve ganglia, some of the cells always 
retaining their nervous character. Other cells, because they stain brown 
or yellow with chromic salts, are called chromaphile or phaeochrome 

Urogenital System 


cells. (Fig. 464.) These are usually quite closely related to blood vessels. 

The interrenals arise from the 
epithelium of the coelom. There 
is as yet considerable doubt as 
to whether they are connected 
with pro- or meso-nephros, or 
whether they are totally distinct 
in origin. They arise as isolated 
clusters, or bands, of cells near the 
dorsal margin of the mesentery. 
Sometimes they are bilaterally 
symmetrical, and in the lower 
vertebrates may extend through- 
out the entire length of the 
coelom in the early stages. 

Both interrenals and supra- 
renals are separate in the fishes. 
The interrenals are the more com- 
pact of the two and lie between 
the excretory organs of the two sides of the body. 

The suprarenal tissue forms the medulla of the adrenals from a 
series of tubules through which the blood from the suprarenal artery 
circulates before it is carried away by the vein. The adrenals are closely 
associated with the Wolfifian bodies in amphibia (Fig. 351), either being 
attached to the inner margins (urodeles), or forming yellow stripes 
(anura) on the ventral surface. In the reptiles they are lobulated bodies 
near the gonads. 

It is from the medullary portion in mammals that adrenalin, some- 
times also called epinephrin, is obtained. This is an activator or 
hormone which acts directly on the muscles and causes an increase in 
blood pressure. 


Fig. 464. 

The phaeochrome system of a just-born 
B, The same in a forty-five day girl, a, 
k, kidney; p, phaeochrome bodies; r, 
s, suprarenal; m, ureter. The connection 

between the bodies and the central portion of the 
suprarenal is shown in A. (From Kingsley after 

Fishes (Fig. 457) : 

The excretory system consists of elongated bodies situated in the 
median dorsal part of the coelom. These bodies are composed of nephric 
tubules which have funnel-like nephrostomes opening into the coelom. 
The functional kidney is a mesonephros. The ovaries and testes (with 
the exception of the teleosts) are sac-like structures which have ducts, 
oviducts, and vasa efferentia developed in connection with the primitive 
nephridial duct, as in other groups. 

In teleosts, there are no vasa efiferentia or true oviducts, for the 
posterior ends of both testes and ovaries are continued into a duct direct; 
the duct from the testes unites with its fellow on the opposite side to 

400 Comparative Anatomy 

empty into either a urogenital sinus or directly to the outside, and the 
one from the ovary takes the eggs direct from the ovary before they 
enter the coelom as in most of the higher forms. 

The eggs of different fishes range from large heavily-yolked eggs 
with chitinous shells, as in the modern elasmobranchs, to the small 
pelagic eggs of many modern teleosts. The eggs pass out through the 
ducts of teleosts as mentioned in the preceding paragraph or through 
abdominal pores as in ganoids and in some Physostomi. 

For the most part, fish-eggs are fertilized in the open water, 
although there are many orders which practice internal impregnation 
and are viviparous. 

Most of the teleosts are dioecious but some are hermaphroditic. 
Serranus, a member of the perch family, is even self-impregnating; 
Chrysophrys is successively male and female; while cod and herring 
often exhibit the hermaphrodite condition, though this is abnormal. 

Dogfish (Eig. 462) : 

The pronephros is never functional as an excretory organ. The 
nephrostomes fuse to form the ostium tubae in the female. 

The pronephric duct splits into both a Wolffian and a Mullerian 
duct. The nephrostomes close in the adult. The anterior end of each 
mesonephros is narrov/ed, and, in the male, this connects with the 
anterior end of the Wolffian duct to form a connection with the testes. 
The epididymis consists of the coiled anterior end of this connection. 

The Mullerian ducts become the oviducts. The oviducts of both 
sides connect with the coelom. The common opening thus formed is 
the ostium tubae abdominale. 

The eggs leave the ovary, pass to the ostium, and are then carried 
backward to a shell-gland. The enlarged portion of the tube forms the 

In the male, the anterior end of the mesonephros forms the epididy- 
mis while the vasa deferentia of both sides unite to form a urogenital 
sinus. There is an oval sperm-sac connected on each side. Fertilization 
is internal. 

The suprarenals are metameric and may be imbedded in the 

Amphibia (Fig. 457) : ' 

The pronephros functions until metamorphosis. The tubules then 
degenerate. In the adult frog and other tailless amphibians, the nephro- 
stomes of the mesonephros separate from the nephridial tubules to join 
with branches of the renal blood vessels so that the coelom is in direct 
connection with the excretory system. 

The Wolffian duct carries the nephridial waste and the same duct 
also acts as the vas deferens in the male just as it does in the dogfish. 

Urogenital System 


Where these ducts enter the cloaca there is an enlargement on each to 
form the seminal vesicle. 

The urinary bladder lies ventral to the cloaca. The eggs ])ass into 
the body cavity and thence into the ostium tubae. 

Fertilization is external in the tailless amphibians, but internal in 
tailed amphibians. The male of the tailed amphibians secretes a sub- 
stance which binds the spermatozoa into little packets called spermato- 
phores. There are various accessory reproductive relations as mentioned 
in the chapter on classification. 

Reptilia and Aves (Figs. 462, 465) : 

The kidneys are metanephric bodies which 
pass their excretion through paired ureters 
directly to the cloaca in the reptiles and 
from here into a urinary bladder which, in turn, 
empties into the cloaca. The pronephros never 
functions, and the mesonephros (always lack- 
ing nephrostomes) may function after hatch- 
ing for a time in some reptiles. In the female, 
the mesonephros, after degenerating, is pre- 
served as the "yellow-body." The male repro- 
ductive organs consist of a pair of testes, a 
pair of much coiled vasa deferentia through 
which the sperm passes to the grooved penis; 
the latter organ being attached to the front of 
the cloaca. The female organs consist of 
paired ovaries and large oviducts provided 
with albuminous and shell glands. The eggs 
when laid are covered with a tough shell ; those 
of reptiles are usually buried in the ground. 
Many reptiles are, however, viviparous. The 
Wolffian duct is the urinary tube. The 

mesonephros functions in both sexes but later degenerates in the female. 

It persists in the male as the vas deferens. 

In birds the left ovary alone remains functional. 

Mammalia : 

Only two pronephric tubules form and these never function. The 
mesonephroi function in foetal life and in marsupials and monotremes 
for some time after birth. Nephrostomes never form except in Echidna. 
In some rodents no glomeruli occur. The kidneys are of the metanephros 
type. They are usually asymmetrical in position, one lying anterior to 
the other. The ureters lead directly to the urinary bladder which is 
formed out of the remains of the allantois. 

The ovaries are never single as in birds. They are very small on 
account of producing minute eggs with little or no yolk. This small 

Fig. 465. 

Cloaca and urogenital 
organs of a turtle, Chelydra 
serpentina, c, c', blind sacs of 
cloaca; cl, cloaca; e epididymis 
and vas deferens; p, penis; r, 
kidneys; re, rectum; s, groove 
on penis; t, testis; u, ureter; 
v,g,, cloacal opening of bladder; 
V, bladder. (From Sedgwick's 
Zoology, after Gegenbaur.) 

402 Comparative Anatomy 

size of ovaries and eggs is well fitted to the habit of uterine gestation. 
The paired oviducts enlarge to form paired uteri, and in some groups 
these unite into a single median uterus. 

The testes at first lie in the body cavity, as in reptiles, and occupy 
positions homologous with those of the ovaries. In most mammals, with 
the exception of monotremes, whales, elephants, armadillos, and a few 
others, the testes descend into the scrotum. The penis of the male 
mammal is homologous with the clitoris of the female. 



THE general muscular system has been discussed in considerable 
detail in the study of the frog, while the development of the mus- 
cles was taken up in the study of embryology. 

It will be remembered that histologically there are voluntary and 
involuntary muscles ; the former are striated, the latter smooth, while 
the heart muscles are a sort of combination of these two. 

The smooth muscles have their beginnings in mesenchyme, and, 
being involuntary, are innervated by the sympathetic nervous system. 
Their action is also much slower than that of the striated muscles. They 
are found in the skin, in the walls of blood vessels, in the walls of the 
digestive canal and in the urogenital system. 

The striated muscles have their origin in the walls of the coelom 
and are of mesothelial origin. They are supplied by the motor nerves of 
the central nervous system. They are all voluntary except those at the 
more cephalic end of the digestive tract. Striated fibers may be found 
in the body walls, in all organs of locomotion, in the head, in the 
diaphragm, and in the cephalic end of the alimentary canal. 

The voluntary muscles arise from the somites (which divide into 
myotomes and lateral plates, after the epimeres have given rise to the 
sclerotomes and dermatomes). 

The myotome grows downward between the hypomere and the skin 
to meet its fellow on the opposite side in the median ventral line. This 
produces 'a completed coat of voluntary muscles which lies beneath the 
skin. The muscle coat is divided into a dorsal and ventral part by the 
horizontal skeletogenous partition (Fig. 423) which intersects the skin 
at the lateral line. The dorsal muscles are called epaxial, and those 
ventral to the septum, hypaxial. 

The muscles originating from the lateral plates in the gill-arch 
region, which move the gill arches, are called visceral muscles. 

The muscles originating from the myotomes are called parietal or 
somatic muscles. 

All muscles except the diaphragm and heart (the heart is always 
included under the circulatory system) are divided into three groups 
known as parietal, visceral, and dermal muscles. 

From the study of embryology it will be remembered that the 
myotomes were cut ofif from the walls of the coelom, each one forming a 
closed sac, the inner wall called the splanchnic layer and the outer the 
somatic layer. The more dorsal cells of the splanchnic layer develop 
many nuclei which can be seen in the interior of the cell in the lower 


Comparative Anatomy 

vertebrates. They are quite close to the surface in the muscle fibers 
of mammals. Each myotome has its splanchnic wall converted into a 
muscle so that there are as many primitive muscles as there were 

The somatic wall of the myotomes does not become muscle but 
changes into mesenchyme from which the corium of the skin develops. 
Some of the mesenchyme protrudes between the various myotomes and 
there forms fibrous connective tissues that later become the ligaments 
which connect the various muscles of a side. 

This primitive muscle segmentation can still be seen in the inter- 
costal and rectus abdominis muscles. 

The myotomes lie close to the level of the notochord and spinal 
cord, but they grow both dorsally and ventrally, working their way 
between the skin and the walls of the coelom to become an actual part 
of the- somatopleure. 

Ventrally, the muscles from both sides grow toward each other and, 
practically, meet at the mid-ventral line. The direct mid-ventral line, 
which is filled with connective tissues, is known as the linea alba. 

In the fishes, the trunk and tail muscles are arranged in myomeres 
which take a ziz-zag course. (Fig. 401.) The muscles are divided hori- 
zontally into dorsal and ventral portions (Fig. 423), the epaxial and 
hypaxial muscles, a line of division which follows more or less closely 
the lateral line. The plates of muscle do not retain their flat ends in the 
adult, but one end becomes conical and fits into a corresponding hollow 
in the next plate. In the tail of the amphibia, epaxial and hypaxial 
muscles are clearly recognizable, but farther forward the hypaxials are 
greatly reduced, and in the amniotes, the reduction is carried so far that 
the epaxial muscles, greatly modified, can only be recognized in the 

cervical and pelvic regions, the 
"tender-loin" being epaxial. 

The developmental conditions 
are more complicated in the head 
than in the trunk. In the head 
region of fish and birds, ten 
coelomic pouches develop, while 
in amniotes the number is appar- 
ently twelve. These are known 
by number, with the exception of 
the most anterior which was not 
known when the numbers were 
applied and is called A. These 
coelomic or head cavities differ 
from the myotomes farther back 
by having no undivided portion 

Fig. 466. 

The head of a dogfish, seen from above with 
the right orbit opened, e., eyeball; o.i., o.s., in- 
ferior and superior oblique muscles; r.e.,, 
r.s., external, inferior, internal, and superior recti 
muscles; s.p., spiracle; //., optic nerve; IV., 
fourth nerve. (From Borradaile.) 

Muscular System 405 

of the coelom below corresponding to the hypomeral zone. This differ- 
ence is possibly due to the existence of visceral clefts in this region. 

Four of these cavities lie in front of the ear of which A disappears 
completely, its cells joining the mesenchyme. The other three give 
rise to the "eye muscles" which move the eyeball. (Fig. 466.) In gen- 
eral, 1, which lies in front of the mouth, gives rise to four muscles, the 
inferior oblique and three of the rectus muscles ; 2, which lies in the 
region of the jaws, forms the superior oblique muscles; while 3, in the 
hyoid region, develops the lateral (external) rectus (in some animals also 
a retractor bulbi). The origin of these muscles explains the distribution 
of the eye-muscle-nerves, as each nerve supplies only the derivatives of 
a single myotome. Several of the other myotomes disappear in develop- 
ment, but the posterior becomes the so-called hypoglossal musculature. 

We have been describing only the origin of the contractile tissue of 
the muscles. There is also a connective tissue to be considered. 

Mesenchyme cells invade the muscle fibers to form envelopes 
(perimysium) which bind the fibers into bundles (fasciculi) ; these in 
turn, are united by other envelopes called fascia. These connective- 
tissue envelopes continue beyond where the contractile tissue leaves 
off to form the cords, or tendons, by which the muscle is attached. The 
more fixed point of attachment is called the origin, the less fixed the 
insertion. Tendons may be of any shape ; such as long and slender, so 
as to allow the muscle to lie in or near the trunk, the part to be moved 
being in the appendage ; or they may form broad flat sheets (aponeu- 
roses). These latter may occur not only at the ends but in the middle 
of a muscle. 

Sometimes parts of tendons ossify, as in the patella or in the ''drum- 
stick" of the turkey. Such small rounded ossifications of this kind are 
sesamoid bones. 

In a few cases, as for example, around the eye and mouth of 
mammals, the parietal muscles are without attachment. Here they 
form rings which are used to diminish the size of an opening (sphincter 

Muscles vary considerably as to shape, size, number of "heads" or 
points of origin, and numbers of contractile portions. 

Muscles are usually arranged in antagonistic groups so that any 
given action may likewise be reversed. We thus have flexors to bend a 
limb and extensors to straighten it ; elevators to close the jaw, depressors- 
to open it, etc. 

It is rather difiicult to trace exact homologies. The test usually con- 
sidered best is to trace the nerve supply, for every muscle derived from 
a given myotome is innervated by branches of the nerve which also 
originally connected with that segment. A further test is the origin 
and insertion. The action of a muscle is of little value in a test for 

406 Comparative Anatomy 

A difficulty in the drawing of conclusions from specimens before one, 
comes from the fact that a muscle may split into various layers either 
longitudinally or transversely, and some even, though entirely different 
in origin, may fuse together, while others, either in part or in whole, 
may degenerate and disappear entirely. Should one take nerve supply 
as a guide, as is usually done, it will be seen that the facial muscles, 
especially those of the higher mammals, have certainly wandered a long 
way from their embryologic origin. 

The names and location of muscles of the frog should be thoroughly 
reviewed at this point. 

In the higher vertebrates the anterior spinalis differs from the frog 
by being divided into several rectus capitis muscles which connect the 
first vertebra with the skull. 

The longissimus dorsi group lie on each side of the vertebral spines 
in the angle between spinous and transverse processes and extends from 
the pelvis to the head. This group is made up of a longissimus dorsi 
proper in the lumbar region, an ileo-costalis (inserted on the dorsal part 
of the ribs), and a longissimus capitis along the side of the neck to the 
temporal region of the skull. 

The muscles of the appendages are divided into intrinsic and extrin- 
sic groups. The former have their origin in or on the appendicular skele- 
ton itself; the latter have their origin on the trunk or axial skeleton and 
their insertion on the girdle or base of the limb. Intrinsic muscles, 
therefore, move parts of the limb ; extrinsic move the limb as a whole. 
Muscles are often divided according to their action as already seen. 
Protractors draw a member forward ; retractors pull it back against the 
body ; levators lift it, and depressors pull it down ; flexors bend a limb 
or its parts ; extensors straighten it and rotators turn it upon its axis. 

Some of the more prominent muscles are as follows : 

Levators : 

trapezius (for fore limb), 
levator scapulae (for fore limb). 


pectoralis (for fore limb), 
serratus anterior (for fore limb). 

Protractors : 

pectineus (for hind limb), 
adductors (for hind limb), 
sternocleidomastoid (for fore limb), 
levator scapulae anterior (for fore limb). 

Retractors : 

pyriformis (for hind limb), 
pectoralis minor (for fore limb), 
latissimus dorsi (for fore limb). 

Muscular System 


The pubofemoralis draws the hind limb toward the mid-line while 
the gluteus muscle acts as a retractor and elevator. 


As already stated, the gill-bearing vertebrates develop a special 
system of muscles in connection with the visceral arches which are used 



branch of facial nerve 
parotid duct 


subniaxillary gland 

first deltoid 
lateral head of the triceps 
long head of the triceps 

pcctoralis major 

serratus ventralis 


parotid gland 

rhomboideus capitis 
external jugular vein 
levator scapulae ventralis 

pectoralis minor 
anterior trapezius 

posterior trapezius 

— latissimus dorsi 

external oblique 

Fig. 467. 
A, Superficial muscles of the cephalic part of the tailed ainphibian Salamandra 
maculata. a, anconeus; bi, humero-brachialis inferior (biceps); he, levator 


Comparative Anatomy 

for the purpose of opening and closing the clefts and also the mouth. 
In the higher forms many of these muscles have disappeared although 
some do retain their connection v^ith the hyoid. 

Visceral muscles are often divided into tw^o sets according to their 

constrictor pharyngis 

molar gland 
parotid gland 
submaxillary gland 
lymph glands 


cut edge of 
Weeps brachii 
teres major 


long bead of 
the triceps 

transversus costarum 
serratus ventralis 

pectpralis major 

cut edge of 


midventral line 

rectus abdominis 
latissimus dots! 

external oblique 

Fig. 468. 

Ventral view of the anterior part of a cat to show the muscles. All dermal 
muscles have been removed. Superficial muscles on the right side, deeper layer of 
muscles on the left side, after removal of the pectoral muscles, sternomastoid, 
mylohyoid, and digastric. The nerves and blood vessels which cross the axilla have 
been omitted. The epitrochlearis is also called extensor antibrachii. (From Hyman's 
"A Laboratory Manual for Comparative Vertebrate Anatomy," by permission of 
the Chicago University Press.) 

scapulae; cue, cucularis; dtr, dorso-trachealis; dg, diagastric; ds, dorsalis scapulae; 
eo, external oblique; Id, latissimus dorsi; tti, petro-tympano-maxillaris (masseter); 
mh, mylohyoid; mh.p, mylohyoid posterior; pc, pectoralis; ph, procoraco-humeralis; 
ra, rectus abdominis; spc, supracoracoid; sth, sternohyoid. (From Kingsley after 

B, Lateral view of the anterior part of the rabbit to show the muscles. The 
head is turned slightly so as to give a ventral view of the throat. All dermal 
muscles have been removed. (From Hyman's "A Laboratory Manual for Com- 
parative Vertebrate Anatomy," by permission of the University of Chicago Press.) 

Muscular System 


derivation, as some develop from muscles which originally ran in a 
transverse (circular) and others from muscles which ran in a longitudi- 
nal direction. 

The epibranchial muscles, the sub-spinals and interbasales (which 
lie in the dorsal part of the branchial region), and the coraco-arcuales (in 
the ventral or hypobranchial half) are derived from the circular group. 
The most anterior of this circular group (Figs. 467, 468) are those which 
open (digastric or depressor mandibulae) or close (adductors) the 
mouth, and the mylohyoid which extends between the two rami of the 
lower jaw. There are usually several adductors, known as masseter, 
temporalis, or pterygoideus, named after the parts of the skull which 
serve as their origin. 

The longitudinal muscles are largely confined to small slips which 
pass from one arch to the next. These muscles undergo considerable 
variation in amphibians. In the amniotes there is also much variation, 
but some of them are reduced on account of the loss of branchial respira- 
tion with a consequent degeneration of the parts connected with it. 
The most noticeable visceral muscles, therefore, in the higher groups 
are those connected with the opening and closing of the mouth. 

Up to this point all muscles mentioned have had a direct connection 
with the skeletal system. With an increasing degree of development 
there develops a dermal musculature. Here the muscles are inserted 
directly in the skin although they were derived from skeletal muscles. 
Primitive conditions of this kind are found in reptiles and birds and serve 
to move scales, scutes, and feathers. This musculature attains its highest 
development in many of the four-footed animals, who use it to twitch 



Fig. 469. 

Cross-sections through the thigh of A, rabbit, and B, cat, to show the location 
of the muscles. Black spots are nerves, small circles, blood vessels, a, greater 
saphenous nerve, artery, and vein; b, peroneal nerve; c, tibial nerve; d, sciatic 
vein; e, fejnpra] nerve, artery, and vein; /, sciatic nerve. (From Hyman after 

410 Comparative Anatomy 

the skin when insects attack them. In the primates, the platysma 
myoides in the neck and head is the only muscle of the kind. It is 
innervated by a facial nerve v^hich in its primitive condition came from 
the hyoid region. The platysma divides to give rise to such muscles 
as the orbiculares, which close the lips and eyelids, and the muscles by 
which one lifts the lips, nose and lids, and by which some are able to 
move the ears. 



IT WILL be remembered that the nervous system begins its existence 
in the embryonic state by the ectoderm of the gastriila becoming flat 
on the dorsal surface of the embryo. This flat portion, called the 
neural plate, extends practically the entire length of the embryo. It is 
slightly broader at the head end than at the caudal end. The two edges 
of the neural plate become raised slightly and finally meet in the mid- 
line of the dorsal surface to form a tube. The closing of the tube begins 
at the head end and gradually extends backward until the tube has 
become completely closed. (Fig. 262.) 

The neural plate, consisting of ectoderm, folding as it does, 
causes the interior lining of the tube to be ectoderm. This is a fact 
of considerable value in understanding various structures, such as the 
development of the eye. It is also well to remember here that the 
various sense organs, or special organs of sense, as they are often called, 
have to do with such things as touch, sound, taste and light, whose 
stimuli come first to the exterior part of the body. In the lower animal 
forms, such as the earthworms, there are no definite eyes, and yet, light 
rays, when thrown upon any part of the earthworm's body, cause it to 
move out of such light, showing that the animal is sensitive to these 
light rays even though no organ has developed by which any one par- 
ticular spot is specialized to receive more impressions than another 

Now, just as the complete digestive tract develops from a straight 
tube by inpushings and outpushings, so the greater portion of the 
nervous system has developed in a similar manner from the single nerve 
tube which has just been discussed. 

One of the explanations given as to why the nervous system 
develops in the way it does from the ectoderm and on the dorsal surface 
of the embryo, is that remote ancestors of the vertebrates may have 
spent their years upon the ocean-bottom, causing the ventral surface of 
the body to lie in contact with the ground substance and thus serve as a 
protection from attack, while the upper part of the body came in contact 
with substances and animals inimical to it. These vertebrate ancestors 
thus needed a sense-perception-organ for protective and nutritive pur- 
poses. Interesting as this may be, it must be admitted that one of the 
great difficulties with which biologists have had to deal is the fact that, 
in the invertebrates, the nervous system lies upon the ventral surface. 
It is only in the higher forms that it lies upon the dorsal side. Several 
ingenious explanations have been attempted to account for all this but 

413 Comparative Anatomy 

none is satisfactory. Students should appreciate the fact that the com- 
plicated nervous system which controls every movement of the body is 
one of the most highly elaborate protective systems we could possibly 

The brain itself, the head-end of the nervous system, is enclosed in 
a remarkable bony case, while the spinal cord (the continuation of the 
brain, caudad), is encased within the slightly movable but nevertheless 
well fitted vertebrae that make up the spinal column. The brain and 
spinal cord combined are called the central nervous system; this to dis- 
tinguish it from the peripheral nervous system which consists of all those 
nerves arising from the brain and spinal cord. 

As the. central nervous system is composed of an infolding of the 
outer portion of the body, the ectoderm thus infolded into the central 
portion of the neural tube, becomes the sensitive part of the central 
nervous system. This sensitive surface lines the lumen of the neural 
tube, and while this condition remains in all higher forms including 
man, it will be seen in the study of the brain that larger or smaller 
masses of gray matter may migrate to various parts of the brain. 

It will be readily understood that a nervous system of this kind, 
which is well protected by a bony covering, has many advantages over 
mere external tactile-sense spots, such as the earthworm possesses ; 
still, to be of any value whatever, any inner sensory portion must retain 
its connection with the outer portion of the body. It is such connections 
Avhich, when they have definite cells and processes that unite with the 
central nervous system and are grouped together, become special sense 
organs. Such nerve fibers, together with their cells, are known as 
sensory nerves. Sensory nerves must, therefore, carry impulses from 
outer portions to innermost regions, or in other words, from the external 
portions of the body to the central nervous system. 

The purpose of the nervous system is primarily to inform the animal 
of the conditions, good and bad, in the environment, to correlate this 
information, and to regulate the motion so that advantage may be had 
of this knowledge. In those forms of animals which are segmented, that 
is, in which metameres appear, especially when this metamerism is in 
the mesothelium from which the myotomes develop into muscles, there 
are usually one or more pairs of motor nerves going to these segments 
because each muscle must have its own nerve supply. The motor nerves 
carry impulses from the central nervous system to the muscle or organ 
in which they are placed. 

The close association of sensory and motor nerves in the trunk 
region of vertebrates has not been satisfactorily explained. In 
Amphioxus the two kinds of nerves are independent of each other 
throughout their course which tends to show that the vertebrate con- 
dition is not primitive. 

Nervous System 



After the neural tube has formed by a joining in the dorsal mid-line 
of the two folds of the neural plate, the cells on each side of the neural 
tube proliferate very rapidly while those of the roof and floor do not. 
This causes an outgrowth of the two sides so that a fissure (Fig. 470) or 

See- ^wn 

Fig. 470. 
Cross-section of spinal cord. A, "spider" cells; B, "mossy" cells. 

groove is formed on the ventral surface, running the whole length of the 
cord. In fact, the cells on each side have already begun to proliferate 
before the closing of the tube. There is an ingrowth of connective tissue 
and blood vessels on the dorsal mid-line which forms a posterior or 
dorsal septum dividing the dorsal part of the cord into halves. The 
entire lining of the central canal, composed of epithelial cells, is known 
as ependyma and, while no definite nervous cells can be seen, it is 
sensory and remains sensory throughout its entire career. 

The remaining cells on each side develop into two kinds of cells, 
one called neuroglia or simply "glia" cells, which are used to support the 
true nerve cells ; the others form neuroblasts which develop true nervous 
tissue. This latter type of cell must develop a fiber in order to connect 
with other cells and with other portions of the body. These are formed 
by a cytoplasmic outgrowth from the neuroblast itself. Such processes 
may be several feet in length or very short. Some of the little fibers 
produced in this way may extend out from the cord as individual nerves, 

414 Comparative Anatomy 

while others run longitudinally within the cord. Others run on the out- 
side of the cord longitudinally. Those which run along the outer portion 
of the cord are often called the marginal layer because they form a sort 
of envelope of fibers for the neural cord itself. These fibers are medu- 
lated or covered with a white substance, and this white envelope is called 
the white matter of the cord. That portion which lies further toward 
the lumen and is composed largely of cell bodies, constitutes the gray 

In cross-sections of the spinal cord of a higher vertebrate there will 
be seen a portion looking something like the capital letter "H" with a 
central canal in the middle of the cross-bar. The entire substance, which 
looks like the letter "H," is the gray matter of the cord. The dorsal 
upright bars of the "H" form the posterior columns, while the ventral 
uprights form the anterior columns of the cord. Immediately lateral to 
the cross-bar on each side of the cord there is another column known as 
the lateral column. The lateral column differs to a considerable extent, 
not only in its relation but also in its function, from the dorsal and ven- 
tral column. This H-shaped gray matter really divides the white matter 
into three longitudinal tracts called funiculi, formerly also called 
columns. They are known as the dorsal, ventral, and lateral funiculi. 

It will be remembered that the white matter is composed of longi- 
tudinal fibers. It is these longitudinal fibers which make up the various 
funiculi which connect the different parts of the central nervous system 
with each other. It is important to remember that these fibers are not 
all alike, but that those in the dorsal funiculus carry impulses toward the 
brain and are, therefore, called ascending tracts ; w^hile the ventral funicu- 
lus is known as the descending tract in that it carries fibers from the 
brain downward. The lateral funiculi have fibers of both kinds and carry 
impulses in both directions. 

The fibers in each of the funiculi are again grouped into smaller 
bundles or fasciculi, each with its own name. Some of the fibers coming 
from the brain are distributed at different levels along the cord, while 
others, going to the brain, are added to the funiculi at different places. 
The size of the funiculi thus decreases with the distance from the brain. 
Some of the bundles may disappear in the more distal parts of the cord. 

The spinal cord is approximately cylindrical in the higher animal 
forms, but in the lower it is flattened dorsoventrally, the flattening being 
greatest in the cyclostomes. In the lower groups there is also a differ- 
ence in the shape of the gray matter, the H shape being less distinct. 

The cord tapers quite regularly in fishes, from the brain to the 
posterior end, but when legs have developed with an increase of mus- 
culature, the spinal cord becomes enlarged in the regions where the 
nerves for the limbs are given off. Casts of the spinal canal in certain 

Nervous System 415 

fossil reptiles indicate that there was an accumulation of nervous matter 
near the hind legs which exceeded the brain in size. 

The nerves leave the spinal cord at nearly right angles to its axis 
v^hen development begins. Then there occurs an inequality in growth, 
the body increasing more in length than does the cord. As a result the 
more caudal nerves pursue a very oblique course, and in the hinder part 
of the spinal canal of the higher vertebrates they form a bundle of 
parallel nerves, the cauda equina (horse-tail). Often, too, another result 
of the unequal growth may be the drawing out of the hinder end of the 
cord into a slender, non-nervous thread, the filum terminale (Fig. 17). 

Flexures (Fig. 288). 

In the early stages of development, it will be remembered, the head 
end of the developing spinal cord bends forward at almost right angles 
to the main axis, and this first bend is called the primary flexure. The 
second bending occurs at the most caudal end of the medulla oblongata 
and is called the nuchal flexure; it bends in the same direction as does 
the first or primary. The third bend is at a level with the cerebellum 
and is known as the pontal flexure ; it bends in the opposite direction of 
the other two, thus drawing back the fore part of the entire brain to lie 
on top of the more rearward portion. 

The three flexures just mentioned remain throughout adult life in 
all mammals, but even where one or more of the flexures appear in the 
embryonic state in vertebrates lower than mammals, it is seldom that 
more than one or two remain. In reptiles and birds, the nuchal and 
pontal flexures are weakly developed and entirely obliterated in the 

Neuromeres (Fig. 278). 

Many interesting theories have been advanced in times past as to 
whether or not skull and brain portion of animals were merely a con- 
tinued segmented portion of the spinal column and cord. There has 
never been any satisfactory solution of the problem. This much we 
know : during its development the brain does show some traces of seg- 
mentation in a linear arrangement. These segments are called neuro- 
meres, of which eight are well defined. Five lie in front of the ear, one 
corresponds to the ear in position, and two lie behind the ear. 

It is from the first of these segments (though some insist there are 
two here) that the fore-brain arises, as well as the parts which in turn 
arise from it. The second becomes the mid-brain. The third lies in the 
region of the cerebellum. The fourth and fifth lie in the region of the 
more cephalic portion of the medulla oblongata where the trigeminal 
and facial nerves arise. From the sixth, the glossopharyngeal nerve 
arises, while the vagus is directly connected with the remaining two. 

416 Comparative Anatomy 


In examining any brain, one finds, after the bony parts of the skull 
have been carefully removed, a connective tissue envelope lying close to 
the bone. This is called the endorhachis; it is really the periosteum or 
perichondrium of the bony parts and not a true envelope of the brain. 

In the ascending groups of vertebrates v^e find a more complex 
arrangement of brain and spinal-cord-envelopes. It must be understood 
that what is here said of the brain-coverings proper, must also be said 
of the entire spinal cord. 

In the fishes there is but a single covering envelope called the 
meninx primativa. The blood vessels are carried v^ithin this meninx. 
There is an open space betvs^een this meninx and the endorhachis, called 
the perimeningeal space, filled, as are all such spaces, with the cerebro- 
spinal fluid. Tiny strands of tissue pass between the two connective 
tissue layers. 

In the urodeles, and from there on upward in the various phyla, the 
meninx has two layers, namely, the pia mater, which bears the blood 
supply and lies close to the neural cord, and the dura spinalis or dura 
mater. A space between these two layers is called the subdural space, 
while the perimeningeal space is then called peridural. 

In mammals, the outermost layer of the pia mater again separates 
from the pia proper, becoming a delicate arachnoid layer, and the space 
thus formed is called the subarachnoid space. 

In man and some of the higher groups of animals, the dura spinalis 
unites with the endorhachis, obliterating the subdural space, and this 
united sheet of covering is called the dura mater. This dura mater forms 
two strong folds in the mammals, and to a small extent in birds, and 
presses longitudinally into the longitudinal fissures separating the two 
hemispheres of the brain. It is then known as the falx cerebri. The 
other fold presses transversely between cerebrum and cerebellum, form- 
ing the tentorium. Sometimes these folds even ossify and unite with the 


The forepart of the spinal cord becomes constricted in two places 
transversely, forming three divisions, each hollow in the center (Fig. 

Starting with the cephalic end (Fig. 471), the first compartment 
thus formed is known as the fore-brain or prosencephalon. The central 
portion forms the mid-brain or mesencephalon, while the portion extend- 
ing caudally is called the hind-brain or rhombencephalon. 

Cyclostomes are the only vertebrates whose brains remain in this 
simple three-chambered state. In all other forms there are many modi- 
fications of the primitive brain, though no matter how many modifica- 

Nervous System 417 

tions there may be, they all form as ingrowths or outgrowths of this 
primary type. 

The prosencephalon divides into an end-brain, or telencephalon, con- 
sisting of the cerebral hemispheres, and the twixt-brain, or diencephalon, 
consisting of the thalamus and the hypothalamus. Each of these in turn 
again divides, forming the parts enumerated in the accompanying table. 
(Pages 418, 419.) 

The mesencephalon divides into four lobes (in mammals these are 
called corpora quadrigemina but in lower forms of vertebrates, where 
these bodies have not again divided transversely, they are known as the 
corpora bigemina or optic lobes), and the cerebral peduncles. 

The rhombencephalon is made up of the isthmus rhombencephali 
(consisting of the superior cerebellar peduncles, the anterior medullary 
velum, the trigonum lemnisci, and the crura cerebri, the isthmus itself 
connecting mesencephalon and rhombencephalon), the metencephalon 
(consisting of cerebellum and pons), and the myelencephalon or medulla 


The prosencephalon and the mesencephalon together are often 
called the cerebrum. 

The greater part of the telencephalon is made up of the two hemi- 
spheres which are divided by a longitudinal fissure. This fissure is not 
well marked in fishes, but is very distinct in other groups of animals. 
The lateral ventricles are contained one in each hemisphere, while a part 
of the third ventricle (commonly called the foramen of Monro), (Fig. 
303), lies between the two. The corpus striatum is a ganglion mass 
lying upon the floor of the lateral ventricle, while the cortex of the hem- 
spheres is called the pallium. ^ The substance of the hemispheres varies 
to a considerable extent in the different types of animals. In fact, in 
the fishes it is practically all pallium, for there is merely a thin non- 
nervous covering to the ventricles. In reptiles and birds the gray matter 
(nerve cells) is to be found on the ventricular surface, while the outer 
surface is composed of white matter (fibers). In the reptiles there is the 
beginning of a second layer of cells a little distance from the ventricular 
surface. In birds this is still further increased, while in mammals there 
is a complete layer called the cerebral cortex over almost the entire 
surface of the hemisphere. 

As the brain grows in a bony case, it follows that, as soon as the 
brain has grown longitudinally the full length of this case, it must bend 
in the various directions the case lays down for it. Therefore, in the 
mammals, the posterior end of the hemisphere grows dorsally and down- 
ward, and then forward again until that portion of the hemisphere, which 

^The cortex merely means an outer portion, and, in the brain, is usually composed of gray matter, 
while the pallium is merely the outermost covering of the hemispheres, whether composed of gray 
matter or not. 


Comparative Anatomy 

was originally most posterior, has now grown forward until it reaches, 
or at least touches, the olfactory region. The part growing downward 
and then forw^ard grows over a part of the side-wall of the hemispheres 
which portions of the side-wall thus form a little island in the depths of 
the longitudinal fissure. This island is called the insula (of Reil). 

The bending itself of the downward and forward growing parts has 
caused a deep transverse fissure in each hemisphere known as the lateral 
cerebral fissure or fissure of Sylvius. 

Commissura sup. 
Corpus pineale (Epiphysis) | Lamina chor. epithelialis ventr. Ill 

Stelle der Einstiilpung des 
Plex. chorioideus 

Tela chor. ventr. IV 

Hohe des For. inter- 
ventric. (Monro!) 

Commissura ant. 

Recessus optic. Lamina terminalis 

Fig. 47L 

/ Schematic median section of brain of a four month human foetus to show 
the various changes caused by the developing hemisphere. (From Corning after 
Burckhardt.) , , ,, , ^ it -j 

// Diagram of the development of the corpus callosum and septum pellucidum 
in man A shows the hemisphere in outline, ac, anterior commissure; cc, corpus 
callosum- ep epithelial roof of the third ventricle; he, hippocampal commissure; 
It lamina terminalis; o, olfactory lobe; oc, optic chiasma; p, paraterminal body; 
j-' septum pellucidum; vh, vestigial precallosal and supracallosal hippocampus. (From 
Kingsley after G. Elliott Smith.) 

Nervous System 419 

All higher forms of mammals have the brain substance thrown into 
many folds or convolutions known as gyri (Fig. 473). 

The deeper grooves separating the gyri are called fissures, while 
the lesser grooves are known as sulci. This folding permits a great 
amount of cortex, or gray matter, to be provided for; for, it will be 
noted that not only do the tops of each convolution form cortex, but 
also the entire sides of every sulcus. 

The hemispheres are divided into various lobes : frontal, parietal, 
temporal and occipital. The two hemispheres are connected by various 
commissures which must be studied in the actual brain and compared 
with the diagram. 

Following are the chief commissures (Fig. 471) : 

Anterior commissure, in all vertebrates. 

Pallial commissure, dorsal to the anterior. This appears in verte- 
brates from the amphibians upward. 

Corpus callosum (Fig. 471), and 

Hippocampal commissure. These last two are a variation in the 
higher mammals of the pallial commissures in the lower. The corpus 
callosum is developed to a greater extent in man than in other animals 
(Fig. 472). This is explained by the fact that in no other animal does 
mentality reach so high a state of development as in man, and, because 
the cerebral hemispheres are the seat of mentality, it follows that much 
greater connection between the cortex of the two hemispheres is needed 
in man than in other animals. There is a thin translucent membrane 
between the body of the corpus callosum and the fornix, known as the 
septum pellucidum, which leaves a slight cavity between the two septa 
of each side. Formerly this cavity was called the fifth ventricle. It has 
no connection whatever with any of the true ventricles. 

Two tracts of nervous matter run back on the medial side of either 
hemisphere, from the olfactory lobe to the hinder end of the cerebrum. 
One of these is the hippocampus, which passes dorsad, and the other 
is the olfactory tract, which goes ventral to the foramen of Monro. These 
two and the associated olfactory substances make up practically all of 
the so-called archipallium in the lower vertebrates, for in these the whole 
cerebrum really is accessory to the sense of smell. In mammals and 
possibly as low as the reptiles, a part has been added to receive impres- 
sions from other somatic senses. This is the neopallium wdiich has 
grown out lateral to the, hippocampus and is especially large in the 
higher mammals. In man it forms by far the greater part of the cere- 
brum. Its great development forces the olfactory parts to the medial 
and lower surfaces so that they are exposed to view only by dissection. 
A part of the original hippocampus is then vestigial. 


Comparative Anatomy 

Fig. 472. 

Comparison of Various Types of Brains, A-F (Edtnger) are sagittal sections 
showing structures lying in the median line and also paired structures (e.g., 
pallium) lying to one side of the median line. The cerebellum is black. It is 
doubtful whether the membranous roof in A indicated as pallium is strictly 
homologous with that structure in other forms. In B, Pallium indicates prepallial 
structures, Aq.Syl., Aquseductus Sylvii; Basis mesen., basis mesencephali; Bulb, olf., 
bulbus olfactorius; Corp. striat., corpus striatum; Epiph., epiphysis; G.h., ganglion 
habenulae; Hyp., hypophysis; Infund., infundibulum; Lam.t., lamina terminalis; 
Lob. elect., lobus electricus; L.vagi, lobus vagi; L.opt., mid-brain roof; Med. obi., 
medulla oblongata; Opt., optic nerve; Pl.chor., plexus chorioideus; Rec.inf., 
recessus infundibuli; Rec.mam., recessus mammillaris; Saccus vase, saccus vas- 
culosus; Sp.c., spinal cord; vent., ventricle; v.m.a., velum medullare anterius; 
v.m.p., velum medullare posterius. G and H show the mesial surface of the cerebral 
hemispheres in a low (G) and high (H) Mammal. G, {Elliott Smith, Edinger, 
slightly modified.) The exposed gray matter of the olfactory regions is shaded, the 
darker shade indicating the archipallium (preterminal area and hippocampal 
formation), the lighter shade indicating the rhinencephalon, which consists of the 
anterior and the posterior (principally pyriform) olfactory lobes, and a central region 
made up of the hippocampus and the following gyri: fornicatus, dentatus, uncinatus, 
introlimbicus, fasciolaris, and Andrae Retzii. 

Nervous System 421 

"Beginning in the amphibia and reappearing in the reptiles is a tract 
of fibers on either side, which connects the posterior part of the cerebrum 
(where the hippocampus ends) with the hypothalmus. In the mam- 
mals, by the flexure of the cerebrum, this same band of fibers, here called 
the fornix, is obliged to take a circuitous course. Starting at the hippo- 
campus on the medial side of the temporal lobe, the fornix runs up, then 
forward, below the corpus callosum, and then down, in front of the 
interventricular foramen to end in a protuberance, the corpus mam- 
millare, on the floor of the hypothalmic region." 

Headward, on the dorsal side, the walls become somewhat thick- 
ened, bulging out into a pair of prominences known as the optic lobes, 
or corpora bigemina, in the lower forms of animals, while in the mam- 
mals there are two such pairs of lobes which are, therefore, called corpora 
quadrigemina (Fig. 473). The roof of this region remains comparatively 
thin, but the floor becomes somewhat thicker and forms the cerebral 
peduncle. Connecting the mid-brain with the hind-brain is a short con- 
stricted area known as the isthmus. From here running caudad along 
each lateral wall there is often a groove (seldom, if ever, seen in the 
adult) called the limiting sulcus or the sulcus of Monro. This naturally 
divides the brain and spinal cord from here to the tail-end into a dorsal 
and a ventral half, a fact that is of considerable importance, because the 
entire dorsal area is sensory, while the ventral is motor in character. 
Further, in the study of the central nervous system's development it is 
the dorsal portion in which most of the changes come, comparatively 
few developing on the ventral side. 

The hind-brain is again divided. The part lying cephalad develops 
into the cerebellum or balancing brain (organ of coordination), while 
the caudal end tapers rather gradually and is known as the myelencepha- 
lon or medulla oblongata. The cavity in the hind-brain, most of which 
is located within the medulla, is known as the fourth ventricle, Avhile the 
small lumen which connects the third and fourth ventricle is called the 
aqueductus cerebri or the aqueduct of Sylvius (Fig. 282). 

It will, therefore, be noticed that from the earlier three compart- 
ments of the head end of the brain and spinal cord there have developed 
five brain divisions with four ventricles. All the ventricles form a con- 
tinuous open space throughout the entire central nervous system. 

The roof plate in the region of the cerebellum, which originally was 
quite thick, forces the most cephalic portion of the two dorsal zones 
far apart, so that they then become quite thin and broad, whereas the 
floor plate becomes greatly thickened and constitutes the pyramids which 
pass in front into the cerebral peduncles. 

A comprehensive study of the brain is a tedious and difficult task 
and requires a very thorough going over, and a remembering- of the main 
points in the histology, general anatomy, and physiology of the frog. 
And the task is made the more dif^cult because all the early studies 


Comparative Anatomy 

Medulla oilongnla 


Sulc. frontal. Sulc. front. \Sulc. /? 

Sulc. centralis Sulc. pflst- 

(Rolandi) centralis Sulc. ititerpariet 

Sulc. intermed. 

Fissnrn pnrieto- 

Sulc. intermed. 

Sitlc occipit. 

S'llci occipitale! 
siiperiores et later 

Polus occipitalis 

Trunctti Jissuraf 

lateral. ( /, 

erei'ri Polns tetn- Sulc. temp. Sulc. Sulc. Incisura praeoccipital. 

post. ) poralis sup. trmp. med. temp. inf. 


Cyr. ,upra. , ^^^^^ 

Cyr. angTti. 

descfndens { Ecker ) 


Fig. 473. 
A, Diagram to show development of five secondary brain vesicles. (After His.) 
B, Median sagittal section through brain of man. C and D, dorso-lateral cerebral 
surfaces. C to show fissures and sulci, and D to show gyri. (After Villiger.) 

Nervous System 


were made upon the human brain before our improved stains made it 
possible to understand the finer structure of nerve cells and fibers. The 
result is that the names of the various parts of the brain have been 
derived from fanciful resemblances, often very confusing. 

We shall attempt to study the entire central nervous system in 
terms of function rather than in terms of structure, and the latter only 
in its development, as then, and then only, are we able to place a valid 
interpretation upon our findings. 

Following are several terms without which no progress in this study 
can be made : 

A center is any group of nerve-cells which performs a single func- 
tion. (This does not imply, however, that all of this particular function 
is located in this one center alone. There may be several, or many, 
performing similar functions.) 

It is these centers which form a sort of switchboard for the redis- 
tribution of various nervous impulses. 

Afferent fibers are those which conduct toward the centers. 
Efferent fibers are those which conduct away from a center. 
Peripheral nerves (those running from and toward the central 

system) are naturally mixed 
-^ (SL— — — ^^g^ nerves in that they carry both 

afiferent and efferent fibers. 

Inhibitory fibers are those 
which check an action. 

White matter (substantia 
alba) is that portion of nerve 
fibers covered with white 
myelin sheaths. 

Gray matter (substantia 
grisea) is that portion which 
consists of a mass of nerve-cell- 
bodies uncovered with myelin 
^ ' ^^"'^' sheaths. 

Brain nuclei are the gray 
centers within the brain, which 
are divided in turn into : 

Primary centers which are 
those directly connected with 
the peripheral nerves, either as 
terminal nuclei of afferent 
fibers or as nuclei of origin of efferent fibers, and 

Correlation centers are those in which the impulse received 
is redistributed after meeting with other impulses at a common center. 


J -Cell fioJi^ 



Fig. 474. 
Five types of reflex arcs. 

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426 Comparative Anatomy 

Figure 474 shows the five ways in which impulses are and may be 

Ganglia are those centers similar to brain nuclei, which lie outside 
the brain; some books still use this term interchangeably with brain 

Brain stem (also called segmental apparatus, because it is supposed 
that the primitive type of brain consisted of a mere tube of nerve-cells 
with which the peripheral nerves were connected, a pair passing from 
each segment as in the spinal cord of the higher forms) is that portion 
of the cephalic end of the central nervous system upon which the enor- 
mous cerebral and cerebellar hemispheres develop in all higher forms. 
These latter are then called the suprasegmental apparatus. 

Cerebrum consists of fore-brain and mid-brain, the most cephalic 
part of which develops into the cerebral hemispheres which are again 
divided as seen in the table. 

The pallium in the highest animal forms is the cerebral cortex or 
mantle (Fig. 472), but in the lower forms such as the fish, in which the 
entire hemispheres are a part of the olfactory apparatus, the pallium con- 
sists of the olfactory apparatus and the two tracts of nervous matter 
connecting the olfactory lobe with the hinder portion of the cerebrum. 
One of these tracts, the hippocampus, passes dorsal, and the other, the 
olfactory tract, passes ventral to the foramen of Monro. They lie on the 
medial side of each hemisphere. 

Archipallium is the word now used for the pallium in the lower 
vertebrates where this mantle is concerned practically only with the 
olfactory apparatus. 

Neopallium has, therefore, come into existence as a term to designate 
the pallium of the vertebrates whose brain is not governed entirely by 
its olfactory apparatus, but where impulses from the general somatic 
senses may be adjusted and be redistributed in a great correlation region 
— the cerebral cortex. In the table, the pallium corresponds to this 
neopallium which has grown out lateral to the hippocampus. 

Rhinencephalon (nose-brain). The entire olfactory apparatus 
divides into peripheral and central regions as shown in the table. 

Corpus Striatum (Figs. 472, 473). This is the name given to the 
entire mass of large nerve cells which connect the brain-stem with the 
cerebral hemispheres. It is also called the basal ganglion. It will be 
noted that the corpus striatum thus forms the main portion of the stem 
of the end brain. It is called striated because it consists of masses of 
gray matter separated by sheets of white matter, thus producing 

In the lower forms of vertebrates (Fig. 473), some have this body 
fairly well developed even though there be no cortex, while in reptiles 
and birds, in Avhich there is a small amount of cortex, it is quite highly 

Nervous System 427 

developed. In these animals, the corpus striatum seems to be a reflex 
center of a higher order than the thalamus. 

There is doubt as to the exact function of the corpus striatum. 
Ramon y Cajal thinks that, in mammals at least, this body functions as 
a reinforcement center of the descending motor impulses coming from 
the cortex, as these fibers give off collateral branches when passing 
through the corpus striatum, while the striatum itself sends important 
descending tracts into the thalamus and cerebral peduncle. 

The white matter consists of fibers that pass between the cortex 
and deep parts of the brain-stem, which have no functional connection 
with the striatum itself. These are called projection-fibers, and are partly 
ascending and descending fibers which pass between the thalamus and 
the cortex, and partly descending motor projection-fibers of the cortico- 
spinal or pyramidal tract, cortico-bulbar tract, and cortico-pontine tracts. 

The gray matter of the corpus striatum forms the two nuclei named 
after their respective shapes, the caudate and the lentiform nucleus (Fig. 
475). Most of the projection-fibers pass between these two nuclei in a 
wide band of white matter which is called the internal capsule. These 
same fibers radiating from the internal capsule toward the capsule are 
called the corona radiata. 

The external capsule is formed of a thinner sheet of fibers external 
to the lentiform nucleus. 

Many cases of apoplexies and other cerebral diseases cause hemor- 
rhage and other injuries in the internal capsule, there destroying some 
of the fibers ; therefore, the study of the exact arrangement of sensory 
and motor projection fibers within the internal capsule is of great clinical 

Claustrum is the name given to the thin band of gray matter lying 
between the external capsule and the cortex of the insula (Fig. 475, B). 

Nucleus amygdalae is a small mass of sub-cortical gray matter under 
the tip of the temporal lobe. It forms part of the nucleus olfactorius 

Thalamus (Fig. 475). The middle and larger subdivision of the 
diencephalon ; sometimes even applied to the entire diencephalon and 
called the optic thalamus. 

As all nervous impulses which reach the brain cortex, except those 
that come from the olfactory organs, pass through the thalamus, this 
organ serves as a sort of vestibule for the cortex and probably also as a 
great relay station for the incoming and outgoing nerves. 

It is to be remembered that the optic fibers Avhich occupy the thala- 
mus take up much of that organ, but it should not be called the optic 
thalamus because all fibers to and from the cortex, regardless of whether 
coming from the eye or not, pass through the thalamus. 


Comparative Anatomy 

Nucleus lentiformis 

Capsula interna (pars lenticulo-thalamlca) 

Nucleus caudatus 

Nucleus amygdala 


Capsula interna 

(pars lenticulo- 



-Traetus opticus •'' 

Hypophy- f anterior lobe '" 

eis cerebri \ posterior lob<v-' ^ ^ f 

Tubor cincreum/// 

Corpus mamillare/ / , 

N. oculcmotorius / / 

Basis pcdunculi' / ^ 

Nervus trigeminua (portio major)-''^^'' 

Nervus trigeminus (portio minor)-' ^^ 

N. facialis-']]^^'- 

N. intermedjus"^"'--''' 

N. acusticus^---' 

N. abducens 

N. glossopharyngcu3'<. ._ 

Nervus vagus -j ,,---""' 

Pyiamis ,. — - 
Fasciculus circumolivaria pyramidia 

Commissura anterior 
Stria termmalis 

^Capsula interna (pars 
'^-i: I .^Nucleus caudatus 

^ ^X^Thalamus 

y Corpus geniculatum 

Corpus pineale 
~~"'Cor. geniculatum mediale 
"■^-Colliculus superior 

CoUiculus inferior 

* Lemniscus lateralis 
"Nervus trochlearis 
•-Brachium conjunctivum 


, , Possa flocculi 

r^^^t^^— Crus flocculi 

--Nucleus denta- 
tus cerebelli 

Corpus ponto-bulbare 

Fasciculus spinocerebellarL 
Nervus spinalis 


Corpus Callosum 

Lateral Ventricle 



Globus Pallidus Major 

Globus Pallidus Minor 

Internal Capsule 

Lateral VentriclP 

Tail o| Caudate Nucleus 


Note: H.C'Hippocampai Commissure 
S.A„h,L,F= tracts to shoulder 
arm, hand, le^. foot. 

Fig. 475. 

A, Left lateral aspect of a human brain from which the cerebral hemisphere 
(with the exception of the corpus striatum, the olfactory bulb and tract, and a 
small portion of the cortex adjacent to the latter) and the cerebellum (excepting 
its nucleus dentatus) have been removed. The brain stem (segrnental apparatus; 
palaeencephalon) includes everything here shown with the exception of the strip 
of cortex above the tractus olfactorius and the nucleus dentatus. Within its sub- 
stance, however, are certain cortical dependencies (absent in the lowest vertebrates), 
which have been developed to facilitate communication between the brain stem and 

Nervous System 429 

Two parts of the thalamus are to be noted. The ventral portion 
contains chiefly motor coordination centers. In man, this portion is not 
w^ell developed and is there called the subthalamus, which is often con- 
fused with the hypothalamus. 

The dorsal portion of the thalamus is again divided into two 
portions : 

(1) The primitive sensory reflex centers, principally in the medial 
group of thalamic nuclei. 

(2) The regular cortical vestibule which forms the lateral nuclei. 
These lateral nuclei are sometimes called the new thalamus (neothala- 
mus) to distinguish them from all other portions of the thalamus, which 
other portions are then called the old thalamus (palaeothalamus). 

In man, the new thalamus makes up by far the greater portion of 
that organ. This portion includes "the lateral, ventral, and posterior 
nuclei (for general cutaneous and deep sensibility) receiving the spinal, 
trigeminal, and medial lemnisci ; the lateral geniculate body and pul- 
vinar (visual sensibility) receiving the optic tracts; the medial geniculate 
body (auditory sensibility) receiving the lateral or acoustic lemniscus." 

It will be noted that Professor Herrick, from whom this quotation 
is taken, considers the two geniculate bodies as a part of the thalamus, 
whereas our table calls them the metathalamus. The student will see 
that all these parts are most intimately connected, and classification is 
bound to be arbitrary no matter what pains may be taken to make such 
classification as scientific as possible. 

All the lateral nuclei are connected with the cerebral cortex by 
important systems of fibers running both to and from the cerebral 
cortex. The fibers themselves are called sensory projection fibers, and 
all of them pass through or near the internal capsule of the corpus 

While these lateral nuclei receive the impulses from the somatic 
sensory fibers as well as the deeper sensibility impulses (such as touch, 
temperature, pain, general proprioceptive sensibility, spatial localization, 
etc., termed as a whole the somesthetic group), this latter group is prob- 
ably separately represented in the thalamus, although we have not yet 
the evidence to demonstrate it. 

Each of the chief functional regions in the thalamus is connected 
with a specific region in the cerebral cortex by its own projection fibers, 
the tracts being known as radiations. For example, there are optic 
radiations, auditory radiations, somesthetic radiations, etc. 

The old thalamus, which comprises the more medial thalamic cen- 
ters found in lower forms, such as fish, has little or no cerebral cortex, 

the cerebral cortex. The chief of these are found in the thalamus, basis pedunculi, 
and pons. Compare this with the side view of an intact brain, Figure 473. (From 
Herrick after Cunningham.) 

B, Horizontal section of human cerebral hemispheres. 1, 2, A. H, L, F, etc.. 
Fiber systems. 


Comparative Anatomy 

and seems to retain its function in higher vertebrates. In other words, 
some ''awareness" of what is going on is carried by these medial centers, 
so that the cerebral cortex is not absolutely necessary for the animal to 
be aware of its own action or reaction. This means that the cerebral 
cortex is not necessary for all, though it undoubtedly is for most con- 
scious purposes. 

Professor Herrick says : "The thalamus can act independently of 
the cortex in the case of painful sensibility and the entire series of 
pleasurable and painful qualities ; for the thalamic centers when isolated 
from their cortical connections are found to be concerned mainly with 
affective experience, and destructive lesions which involve the cortex 
alone do not disturb the painful and affective qualities of sensation." 

Hypothalamus. That portion lying immediately beneath the thala- 
mus. A small portion of the primitive neural tube to which the hemi- 
spheres are attached has remained in a primitive state, not changing or 
having any ingrowths or outgrowths. This unchanged portion is called 
the pars optica hypothalami, and, as will be noticed by the table, is a 
part of the end-brain and not of the diencephalon. The hypothalamus is 
an important correlation center for olfactory and various visual impulses, 
including probably the sense of taste. 

Tuber cinereum is the gray eminence forming the ventral portion of 
the hypothalamus. 

Infundibulum is a funnel-shaped extension of the third ventricle, 
passing through the hypothalamus to the end of the hypophysis (the 
pituitary body or gland which lies in the sella turcica). 

Mammillary bodies are a pair of eminences at 
the posterior end of the tuber cinereum. These 
bodies are olfactory centers. 

Metathalamus. The posterior part of the 
thalamus consists of the geniculate bodies. The 
lateral or external one is a visual center in the thala- 
mus and the medial or internal body in an auditory 

Epithalamus. This is formed by the mem- 
branous choroid plexus (which forms the roof of 
the third ventricle), the habenula, and the stria 
medullaris (a fiber-tract which connects the olfac- 
tory centers of the habenula and the cerebral hemi- 
sphere). The habenula itself is a center for the 
correlation of olfactory sensory impulses with the 
various somatic sensory centers of the dorsal part 
of the thalamus. The pineal body, in a very few 
lower vertebrates, is a sense organ, being called a 
"parietal eye" (Fig. 476). In the higher forms, this 

Fig. 476. 

Anlage of the epiphysis 
(pineal gland) and parie- 
tal organ in the lizard 
Iguana. A in a 9 day 
embryo, and B in an 18 
day embryo. Longitudi 
nal section, ep, epiphy 
sis; pa, parietal organ: 
zw.h., wall of the ven 
tricle in the twixt-brain 
(After von Klinkow 

Nervous System 


sensory function hcis 1)een lost, thoug^h it is now su])posed to be an 
organ of internal secretion. 


In all comparative studies of animals, one must observe lower 
forms in order that the simpler arrangement there found may furnish 
an understanding of the more complex adjustment found in the higher 
forms, as these latter, usually, possess parts that the lower forms pos- 
sess, plus something additional. 

In the study of the nervous system, the dogfish is a good laboratory 
example with which to work. It has no cerebral cortex developed into 
immense hemispheres, as in man, which make it sO' difficult to study 
the underlying parts and note their relationship (Figs. 477, 478). 

In fishes there is a regular system of small sensory canals widely dis- 
tributed containing sense-organs somewhat similar to those in the semi- 
circular canals of the internal ear. Their functions are supposed to be 
somewhat between that of organs of touch in the skin and those of 
equilibrium of the internal ear. The water vibrations of slow frequency 
probably make it possible for the animal thus to orient itself. Their 
innervation comes from the VII, IX, and X pairs of cranial nerves. The 



lat. f. ,p, 

1^ cer. 
' 'a. ch- V-' 

.st.//'^' /P-'^-P' r^P'"' fb.b. 

-n. am 

pal.h.'^ / ^^' , 

A semi-diagrammatic drawing of a longitudinal section throuijh a dogfish, 
passing slightly to the right of the middle line., anterior choroid plexus; 
a.f., anterior fontanelle; au., auricle; au.v., auriculo-ventricular opening and 
valve; b.b., hasibranchial cartilage; b.h., hasihyal cartilage; c, centrum; c.a., 
conus arteriosus; cb., cerebellum; cer., cerebrum; cor., coracoid region of the 
pectoral girdle; gr., grooves in which the teeth are formed; i.p., intercalary 
plate; inf., infundibulum; lat.v., lateral ventricle; M.c, Meckel's cartilage; n.a., 
neural arch;, ampullary sense organs; n.sp., neural spine; nch., notochord; 
oes., oesophagus; op.l., optic lobe; /J.r/i./'., , posterior choroid plexus;, pineal 
stalk; pal.b., palatine bar; pm., pericardium; pp.c, pericardio-peritoneal canal;, sinu-auricular opening; s.v., sinus venosus; sp.c, spinal cord; st.. semilunar 
valves; tng., tongue; v., ventricle;, ventral aorta; 3, third ventricle; 4, fourth 
ventricle. (From Borradaile.) 


Comparative Anatomy 

sensory canals just mentioned are called lateral line organs (Fig. 479) 
and are absent in higher vertebrates. 

If Figure 480 be studied carefully, it will be seen that there is a 
quite definite area or center for each group of impressions. 

The acoustico-lateral area is the terminal center of the lateral line 
nerves as v^ell as of the acoustic nerves (VIII pair). 

The general cutaneous area receives impressions from the remain- 
ing general exterior of the body. 

The nerves from the viscera (that is, from the gills, stomach, etc.) 
enter a visceral area. 

The eye is connected v^ith the optic lobe. 

The nose connects with the olfactory bulb and hemisphere. (Some 
writers have considered the olfactory hemisphere an actual portion of the 
brain equivalent to the cerebral hemisphere in man. These olfactory 
hemispheres are, however, only portions of the olfactory apparatus.) 

The important point is to note that definite brain regions are set 
aside for sensory impressions, and to notice that they are all on the 
dorsal surface (except a part of the olfactory centers). 

A. B. 

Fig. 478. 

A, The brain of the dogfish, seen from above, cb., cerebellum; cer., cerebrum; 
m.o., medulla oblongata; olf.L, olfactory lobe; olf.o., olfactory organ; \op., 
ophthalmic branches of fifth and seventh nerves; op.l., optic lobes; p. St., pineal 
stalk; r.h., restiform body; sp.c, spinal cord; st?.n., spinal nerve; thai., thalamen- 
cephalon; 3, 4, third and fourth ventricles; _ //.-F., VII.-X., cranial nerves. 

B, The brain of a dogfish, in ventral view, cer., cerebrum; inf., return limb o£ 
infundibulum, sometimes regarded as the pituitary body; l.i., lobi inferiores; 
m.o., medulla oblongata; olf.L olfactory lobe; olf.o., olfactory organ; op., 
ophthalmic branches of fifth and seventh nerves; sp.c, spinal cord; s.v., lateral lobe 
of saccus vasculosus; s.v.'., median lobe of the same; II. -X, cranial nerves. (From 

Nervous System 


The regions which Professor Herrick understandably calls "nose 
brains," ''eye brains," "ear brains," "visceral brains," "skin brains," etx., 
show the simplest type of the pattern of functional localization of pri- 
mary reflex centers. That is, all these special "brains" or centers show 
that practically all of the parts of the brain (except the cerebellum) have 
a very definite connection with some particular peripheral organs. 

This means that this type of simple brain is concerned, in so far as 
we can tell (with the exception of the cerebellum), only with simple 
reflex action, there being no large centers for the higher type of adjust- 
ment found in the higher vertebrate brains. However, in the higher 
vertebrates, even including man, there is this same type of simple con- 
nection also, but it is obscured by the greatly enlarged correlation cen- 
ters of which the cerebral cortex is the most important. The distinct 
course in neurology given in medical schools deals largely and primarily 
with the histological structure and function of this cortex. 

A and B, Schematic diagrams of sections of the skin. The sections pass 
through the lateral line organs. A, of a Teleost; B, of a dogfish. A^, lateral 
hne nerve; 6^ sensory nerve ending; the asterisk shows the cutaneous orifice; 
the arrows indicate the direction of the stimulus. 

C. Lateral line nerve of a fish, an.l, anastomosis between the anterior 
and posterior portions of the lateral nerve; an2, transverse anastomosis between 
the right and left lateral nerve; buc, buccal branch of lateral nerve; g.l., lateral 
nerve ganglion- mand.ex., mandibular branch of lateral nerve; m.. spinal cord; 
oph.sup., superficial ophthalmic branch of lateral nerve; r.ll., branch which fol- 
ows the lateral line XX cranial nerve (dotted) to show partial fusion with 
lateral nerve. (From Vialleton, A and B after Dean.) 

434 Comparative Anatomy 

Because the cerebral cortex is found only in the higher forms of 
vertebrates and, therefore, is supposed to have developed later in the 
evolutionary scale than the simpler type such as the fish displays, it has 
been called the neencephalon in contradistinction to the fish type of brain 
which is then known as the old-brain or palaeencephalon. 

Another point to note is that the "ear brain," the *'skin brain," and 
the "visceral brain" are all contained in the rhombencephalon. In fact, 
the "stem" of the rhombencephalon (also called the segmental portion) 
is made up of these sensory "brains" and their corresponding motor 

This is also true in the higher forms, and the cerebellum (in man, 
the pons also in a sense) are suprasegmental extensions. 

In both lower and higher forms, the "eye brain" includes the retina 
of the eye, the optic nerve, and a part of the roof of the midbrain. In 
fish only a few fibers from the optic nerve pass to the thalamus, but in 
the higher forms, the number of fibers to this portion are many, in fact 
so many, that the entire thalamus, as stated, is often called the "optic 

In the fish there are no true cerebral hemispheres, the seemingly 
similar organs are hemispheres of the olfactory tract (with the exception 
of the very small "somatic area" which becomes the corpus striatum 
and cerebral cortex in the higher forms). The olfactory apparatus of 
the fish also embraces the entire epithalamus and hypothalamus. 

It follows, from all that has been said, that no nervous impulses can 
enter the cortex without passing through the reflex centers of the brain- 
stem first. The brain-stem, therefore, must have all the fibers lying 
within it which are to carry such impulses. The suprasegmental por- 
tions are, therefore, correlation, coordination, and readjustment centers. 


The twixt-brain or inter-brain lies directly in front of the posterior 
commissure. Still further to the front it is bounded by the velum trans- 
versum above and the lamina terminalis below (Fig. 282, A, C). The 
cavity in the center is a portion of the third ventricle which extends to 
the optic chiasma. The fiber tracts running from the cerebral hemi- 
sphere backward pass into the side walls. Those lying in the dorsal 
region go through the thalamus where there is a large nerve center. The 
ventral tracts are the cerebral peduncles already mentioned. Directly 
above and in front of the thalamus is the epithalamus which also con- 
tains a nerve center known as the habenula. The hypothalamus, lying 
as its name implies below the thalamus, consists of the tuber cinereum 
in front and the mammillary bodies behind. Both the epithalamus and 
the hypothalamus bear a relation to the sense of smell and are, therefore, 

Nervous System 


developed to a greater extent in all lower animals in which this sense 
is highly developed. Directly behind the velum transversum is the 
superior, or habenular, commissure. 


It is customary to call various parts developed in the roof plate of 
the primitive fore-brain epiphysial structures (Fig. 483, e). Just where 
the cerebral hemispheres and the twixt-brain meet there is a little fold 
of epithelium, already mentioned, called the velum transversum, hanging 
from the roof of the cerebrum. Directly in front of this is a little choroid 
plexus called the paraphysis. The other epiphysial structures belong 
to the twixt-brain and consist of a parietal organ and the pineal gland 
(Fig. 476). Both of these arise from the roof of the twixt-brain 
between the habenular ganglion and the posterior commissure 
where twixt and mid-brains meet. Sometimes they develop as 
a single outgrowth and sometimes as distinct structures. The more 
anterior of the tw^o is the parietal organ or eye. The posterior is the 
pineal gland, also known as the epiphysis. These two organs, although 
varying in the different vertebrates, are usually always present. The 
parietal organ in at least one group of lizards extends on a slender stalk 

Fig. 480. 


A, Side view of the brain of the dogfish Mustclus caiiis. (After Ilerrick.) 

B, Longitudinal section of brain of Trout, aq, aqueduct; bo, bulbus olfactorius; 
ca, ch, ci, cp, anterior, horizontal, inferior, and posterior commissures; cc, central 
canal; cl, cerebellum; cs, corpus striatum; /;, hypophysis; i, infundibulum; iv, 
trochlearis nerve; cc, optic chiasma; p, pallium'; pi, pinealis; sv, saceus vasculosus; 
tl, torus longitudinalis; to, tectum of optic lobes; v, velum transversum; v^, v*, 
ventricles; vc, valvula cerebelli. (From Kingsley after Rabl-Riickhard.) 

436 Comparative Anatomy 

actually passing out of the skull and forming a sort of median eye on 
the dorsal surface of the head. In those vertebrates in which the parietal 
organ does not appear at all, the pineal gland seems to shov^ tracts of 
structure similar to the parietal organ when it does become an eye. It 
will be remembered that the brow-spot seen on the frog is really the 
spot where the pineal gland began growing toward the exterior of the 
body but was cut off by the developing skull. 

It is interesting to note that, notwithstanding the close relations of 
the pineal and parietal organs, the former receives its nerve supply from 
the posterior commissure, while the parietal organ is connected with the 
superior commissure of the twixt-brain. All of these structures in the 
higher vertebrates are completely covered by the cerebral hemispheres 
growing backward over them. In many of the extinct reptiles there are 
large parietal foramina, and it is supposed that these animals, therefore, 
had well developed parietal or pineal organs. Directly behind the lamina 
terminalis there is a chorioid plexus located in the fourth ventricle. This 
comes from the roof of the brain in this region, and a part of it invades 
the third ventricle, while another part, the inferior plexus, sends branches 
through the interventricular foramina into the lateral ventricle. This 
provides a blood supply to the interior portions of the cerebral hemi- 

A funnel-shaped protrusion from the floor of the diencephalon may 
be seen posterior and ventral to the optic chiasma, known as the infun- 
dibulum. This connects with the pituitary body, or hypophysis, which 
latter organ has developed from the mouth region. It is encased by the 
developing skull in a little bony case of its own, called the sella turcica 
(Turkish saddle). The epithelium of the mouth, from which the hypo- 
physis springs, remains connected for a time to that organ, and its point 
of ingrowth into the brain is called Rathke's pocket (Fig. 301, 1). It 
will be noticed that the pituitary body grows upward from the oral 
cavity just mentioned, while the infundibulum grows downward from 
directly behind the optic chiasma to meet it. There are really two parts 
to the pituitary body, both rich in blood and lymph vessels. The organ 
is known as a gland of internal secretion. Its action is supposed to be 
connected with the fat-storing powers of the animals ; sometimes there 
is to be found a postoptic commissure connecting the ventral parts of the 
brain in this region. 


The mesencephalon or mid-brain, as already stated, does not change 
very much from the way it appears in the embryo. On the dorsal sur- 
face there are two lateral swellings, the optic lobes. In mammals these 
are transeversely divided and are called the corpora quadrigemina. If 
they do not divide transversely, they are called corpora bigemina. Each 
optic lobe is connected with fibers from the eye on the opposite side 

Nervous System 437 

to which it itself is located. In fishes, the ventricle of the mid-brain is 
quite large and extends into the optic lobes, but in the higher groups 
of vertebrates, the ventricle is reduced to a very small opening or 
aqueduct. At the anterior end of the dorsal body of the mid-brain, a 
band of nerve fibers crosses from one side to the other. Any such cross 
connections are called commissures. These connect the two sides of the 
central nervous system. Cross fibers of this kind are very numerous in 
the spinal cord and there are also several in the brain. The one just 
mentioned is called the posterior commissure. 


The cerebellum, or metencephalon (Figs. 472, 481), is the coordi- 
nating organ growing behind the two cerebral hemispheres. The isthmus 
which connects the mid-brain and hind-brain lies directly in front of the 
cerebellum. The cephalic anterior wall of the cerebellum meets with the 
isthmus to form a transverse fold, known as the anterior medullary 
velum (valve of Vieussens), which dips into the fourth ventricle. The 
median ridge of the cerebellum is known as the vermis. This is the only 
part of the cerebellum which the lower vertebrates possess. In some of 
the higher reptiles and birds, however, a small outgrowth occurs on each 
side called the flocculus, and it is between the flocculus and the vermis 
that the cerebellar hemispheres develop in the mammals. This pushes 
the flocculus ventrad. 

Quite a number of fibers grow from one side of the cerebellum to the 
other on the ventral side of the brain stem. This forms a large transverse 
band called the pons or bridge. The lower vertebrates have only a few 
fibers of this kind so that the pons is very narrow in them. There is a 
groove or tract running longitudinally from the cerebellum to the mid- 
brain along the side of this pons and these lateral tracts are called 
anterior peduncles, while the central or median tract is called the middle 
peduncle or brachium pontis. The origin in the cerebellum of the 
anterior peduncle is called the nucleus dentatus. 

ytliim meduUare 



Fig. 481. 
Human cerebellum viewed from below and in front. (After Villiger.) 

438 Comparative Anatomy 


This is a large swelling between the cephalic end of the spinal 
cord and the brain proper. Various narrow centers appear in the ven- 
tral side of the floor serving as centers by which and through which 
efferent, or outgoing, fibers are redistributed to other nerve cells. The 
head end of the medulla, by being expanded, forces the various fiber 
tracts of the dorsal funiculi, as well as of the dorsal part of the lateral 
funiculi, over to the side of the cerebellum where they enter, bending 
abruptly inward and forming a cord called the corpus restiforme, also 
known as the inferior cerebellar peduncle, on either side. The rest of 
the fibre tract forms a pair of bands, called pyramids, on the ventral sur- 
face of the medulla which extend cephalad beneath the mid-brain. These 
extensions are called the cerebral peduncles or crura cerebri. They are 
easily found in the lower vertebrates, but in mammals the pons makes 
them difficult or impossible to see. 


While the brain is supplied by blood vessels distributed over the 
outer surface, extensions from the outer vessels push the roof and floors 
of most of the fore- and hind-brain before them, into the ventricle of 
these two regions, very much on the same principle as an outgrowth of 
the digestive tract, such as the liver, pushes its peritoneum-covering 
before it. These foldings of the plates are called telae chorioideae, or 
chorioid plexuses, and it is through these that the nourishing blood 
passes by osmosis into th^ ventricle and into the inner surfaces of the 
brain. It is practically impossible to remove the brain and have the 
fourth ventricle complete. Usually the chorioid plexus of this fourth 
ventricle is torn away because it is very thin in this particular region. 
The large open surface, or cavity, which one sees, when this has been 
torn away, is called the fossa rhomboidalis. 



The brain is extremely small, hardly as large in diameter as the rest 
of the neural tube. There are but two pairs of cranial nerves, which 
have been called olfactory and optic, but in so reduced a brain, homolo- 
gies are uncertain. The sense organs consist of a median olfactory fun- 
nel opening into the neurocoele, a median eye-spot (not sensitive to 
light) on the anterior end of the brain, representing probably a rudiment 
of paired eyes. The notochord extends the entire length of the body, 
projecting in front of the brain. This may mean that the brain has 
retreated from its primitive anterior position. There is no cranium. 

Nervous System 



The brain is small but typically vertebrate in structure. The vagus 
nerve is not included in the cranial region. In the myxinoids, a groove 
runs the entire length of the dorsal surface. There are four pairs of 
lobes: (1) olfactory, (2) cerebral hemispheres, (3) mid-brain, and (4) 
cerebellum. The nasal capsule is enormously developed. The eyes are 
degenerate and without muscles or nerves. There is only one semi-circu- 
lar canal in the inner ear. In the lampreys, the cerebral hemispheres are 
distinct and a band-like cerebellum is recognizable. Eyes are well devel- 
oped with both muscles and nerves. There are two semi-circular canals 
in the ear, a condition intermediate between that seen in the myxinoids 
and that in the true fishes, where three canals are always present. 

The flexures are never very well marked and disappear entirely in 

the adult. 


The olfactory organs are paired and end blindly, not communicating 
with the pharynx as in terrestrial animals and hagfishes. The auditory 
organs are entirely internal, and have no communication with the 
exterior. They serve largely as organs of equilibration, though they also 
receive vibrations. The eyes are much like those of other vertebrates, 
except that they are lidless and have spherical lenses of short range 
vision in the water. The brain is small and shows no fissures. Never- 
theless, it has all the characteristics of the vertebrate brain, though there 
are but ten cranial nerves (Fig. 482). The spinal cord is like that in 

other vertebrates. 


Although the brain is very small and compact, it is larger in propor- 
tion to body size than that of the cyclostomes. The most striking feature 
is the large size of the olfactory lobes and the slight development of the 

Fig. 482. 

Cranial Nerves of the Fish. (Schematic.) ev., spiracle; mand., 
mandibular branch of the V ; max., maxillary branch of the V ; m.t., 
masticator branch of V ; m., neural cord;, deep ophthalmic branch 
of the V; ph., pharyngeal branches of branchial nerves; pot., post-trematic 
branch; pt., pre-trematic branch; pl.c, cervico-branchial plexus; r.pal. VII, 
palatine branch of the VII ; r.pal. IX, palatine branch of the IX; sp, spinal 
nerves; sp.o., spinal-occipital nerves; V to X, pairs of the corresponding 
cranial nerves; 1 to 4, branchial slits. (From Vialleton.) 


Comparative Anatomy 

intercerebral fissure. The cerebral hemispheres are well defined, the 
cerebellum is large, and overlaps anteriorly a part of the optic lobe, and 
posteriorly a part of the medulla oblongata. The corpora restiformia 
are large folds on each side of the cerebellum in front and lateral to the 
rhomboid fossa. The region of the thalamencephalon, from which the 
optic nerve springs, is comparatively small and slender. The spinal cord 
is typical and enclosed within cartilaginous neural arches. The dominant 
sense of the dogfish is olfactory ; the sense organs consist of large con- 
voluted invaginations in close contact with the olfactory lobes of the 
brain. The eyes, although small and probably not especially keen- 
sighted, are well developed and connected within the brain by a rather 
slender optic nerve. The auditory organs are enclosed in cartilaginous 
capsules and consist of three semi-dircular canals, a utriculus, and a 
small simple sacculus. The lateral line sense organs a're in grooves of 
the skin not completely closed. They divide into several branches in 
the head region, one above and one below the eye, and some in the 
hyomandibular region. 

The vertebral column is not very compact. The vertebrae are often 
without a centrum or, if a centrum is present, it is an arch-centrum. The 
nasal tract has no naso-oral groove. It opens by separate nares. The 

brain has a much reduced cerebrum 
with all olfactory lobes. The pal- 
lium is usually non-nervous, caus- 
ing the cerebrum to consist largely 
of the corpus striatum. The cere- 
bellum is larger than a surface view 
shows, because a great portion pro- 
jects into the ventricle. 


The cerebrum is larger than the 
optic lobes, while the olfactory bulb 
is separated from the cerebrum by a 
long tract. The various brain parts 
are quite distinct. In the tailless 
amphibia, the two halves of the 
cerebrum are secondarily connected 
by a transverse band behind the 
olfactory lobes so that a gap is left 
farther back. 

The telencephalon is larger than 
in fishes because the pallium is in- 
vaded with nervous matter on the 
inner side. There is no true cortex. 

Fig. 483. 

Side and dorsal views of young Alligator. 
c, cerebrum; cl, cerebellum; e, _ epiphysial 
structures; h, hypophysis; i, inf undibulum ; ol, 
optic lobes; JI-XII, cranial nerves. (From 
Kingsley after Herrick.) 

Nervous System 


The optic lobes are large and the pineal gland reaches the cranial 
roof in the tailless amphibia. In the gymnophiones there is a pontal 
flexure which brings the pituitary gland beneath the medulla oblongata. 


There is an advance in the nervous system beyond the amphibia. 
The cerebral hemispheres are larger and the cerebellum more complete 
and a cortex is developed. Something of both pontal and nuchal 
flexures is retained. There may be a beginning of a temporal lobe. A 
parietal eye is well developed in lacertilia. It is rudimentary in other 
groups. The olfactory lobes are merged in the hemispheres. The eyes 

a. c. y. 


Fig. 484. 

A. The brain of a rabbit, seen from above with part of the right cerebral hemisphere cut 
away. a.c.q., Anterior corpus quadrigeminum;, anterior choroid plexus; ch, cerebellum; 
cerJt., cerebral hemisphere; crt., cortex; fl., flocculus; fr.l., frontal lobe of cerebral hemisphere; l.v., 
lateral ventricle; lat.L, lateral lobe of cerebellum; m.o., medulla oblongata; occ.L, occipital lobe of 
cerebral hemisphere; ol.b., olfactory bulb;, optic thalamus; p.b., pineal body; p. e.g., posterior 
corpus quadrigeminum; par.L, parietal lobe of cerebral hemisphere; r.3, roof of third ventricle; 
sp.c, spinal cord; Sy.f., Sylvian fissure; tp.l., temporal lobe of cerebral hemisphere; ver., vermis. 

B. The brain of a rabbit from below,, corpus albicans; fl., flocculus; fr.l., frontal lobe 
of the cerebral hemisphere; hip. I., hippocampal lobe; m.ob., medulla oblongata; ol.b., olfactory bulb; 
ol.t., olfactory tract; p.V., pons Varolii; pit., pituitary body; rh.f., rhinal fissure; Sy.f., Sylvian 
fissure; tp.l., temporal lobe of the cerebral hemisphere; 1 1. -XI I., roots of the cranial nerves. (From 

are small, the pupil round, and the iris unusually dark in color. The 
thalami develop so far as to reduce the third ventricle to a narrow slit, 
even causing two edges to unite. This forms the soft commissure, or 
intermediate mass, of the mammalian l)rain. The sense of hcarino- is 


Comparative Anatomy 

not very acute. The tympanic membrane is thin and exposed, and is 
connected with the auditory organ by a slender columellar bone. The 
sense of smell is the keenest of the senses in the turtle, both in the air 
and in the w^ater. In correlation w^ith the keen olfactory sense, the 
olfactory lobes of the brain are highly developed. In the crocodile (Fig. 
483), the brain is decidedly advanced in structure for a reptilian brain. 
The large cerebral hemispheres are especially noteworthy. The tym- 
panic membrane is sunk in a pit. This is a tendency that is carried 
much further in the birds and mammals. 


The brain is very short and broad ; the cerebrum is large but not 
convoluted. The cerebellum is very large and complex. All three 
flexures are partially retained throughout life. The optic lobes are well 
developed. The olfactory lobes are rudimentary, indicating a poor sense 
of smell. The olfactory epithelium is poorly developed, and the sense of 
taste is almost as poorly developed as the olfactory sense. The inner ear, 
especially the cochlea, is more complex than in reptiles. The eye of 
birds is large and highly organized, probably keener than that of any 
other animal. Sclerotic plates cover the eyeball. A fan-shaped pecten 
(Fig. 490) of unknown function is inserted in the vitreous humor. 


Fig. 485. 

Nerve end-organs. A, longitudinal section of tactile papilla, containing a Meissner's corpuscle. 
B, Section through a terminal corpuscle (end-bulb of Krause) from the conjunctiva. C, Section of a 
Pacinian corpuscle. The nerve fiber, n,fn, enters the capsule through the channel /, and has its 
terminal branches at a. (A. C, from Ranvier; B, from Dogiel.) 


It appears from existing remains that some archaic mammals did 
not have a more highly developed brain than reptiles. Modern mammals, 
however, especially the higher groups, have a brain that is much more 
highly developed than that of all other forms. 

In these higher groups the brain is relatively large (Figs. 472, 484), 

Nervous System 443 

the cerebral hemispheres showing- the greatest increase. The increase 
is practically confined to the pallium (neopallium). 

There is an elaborate system of commissures to connect the two 
sides of the brain, the corpus callosum being the most important. In 
fact, the corpus callosum is largest in the highest groups. 

In the lower animals, the olfactory lobes lie at the tip of the cere- 
brum, but in the higher forms the pallial increase pushes the frontal 
lobes forward so that the olfactory lobes are brought to the lower 
surface and are separated from the cerebrum proper by a rhinal fissure 
on each side. 

The olfactory tract and the hippocampal tract connect the olfactory 
lobes with regions farther back, but in man the hippocampal tract is 
largely rudimentary, the corpus callosum acting as the great connecting 

The great numbers of fibres from the increased pallium form the 
corona radiata which connects the cortex with the more posterior 
portions of the brain. And, as connection is made through the thalami, 
the thalamic regions become greatly enlarged, extend into the third 
ventricle, and reduce that to a mere slit. Where the two walls come in 
contact, the intermediate mass is developed. 

The mesencephalic lobes are four in number, and are called the 
corpora quadrigemina, only the anterior pair are connected with the optic 
nerves, the posterior pair being connected with the sense of hearing. 

An important point in the understanding of certain brain structures 
is the knowledge that the pallium causes a folding so that the original 
postero-ventral end of the cerebrum, lateral to the pyriform lobe, is 
pushed below and to the outside of the lateral parts of the hemispheres, 
the fissure of Sylvius marking the place of folding. It is at the bottom 
of this fissure that the island of Reil is found. This is only the covered 
part of the sides of the hemispheres. 

All higher forms of mammals have the hemispheres arranged in 
many convolutions. This permits an increase in surface without neces- 
sitating a great increase of bulk. However, some animals with less 
mental ability apparently have more convolutions than the more highly 
organized, so that it cannot definitely be said that greater convolutions 
necessarily carry greater mental power. ^ 

The mammalian cerebellum, while better developed than that of 
reptiles, is not so highly developed as that of birds. 

Omithorhynchus (Fig. 472) has the most primitive brain of all 
mammals. It is small, the cerebral hemispheres are smooth and lack all 
convolutions. This animal is aquatic in its habits, living in staonant 

^Echidna has more brain convolutions per body-weight than man. 

444 Comparative Anatomy 

water and feeding chiefly on mollusks, crustaceans, and worms which it 
secures by scooping up the muddy bottom with its bird-like bill. 


It will be remembered that one of the outstanding characteristics 
of living matter is its irritability. Contractility is usually added to 
irritability when living protoplasm is discussed. It has already been 
shown that various functions in the body may be carried on when the 
entire nerve supply to that portion has been destroyed. We may, there- 
fore, say that, while irritability and contractility are essential parts of 
living matter, the irritability which causes contractility need not be 
brought about by a definite system of nerves, although the nerves do 
carry the stimulus from one part of the body to another to coordi- 
nate the various parts and to permit them to work together for some 
common end. 

In all higher forms there are external organs of special sense, such 
as the nose, the eye, and the ear. In some of the lower forms, such as 
the earthworm, we found, that while there are no definite eyes, the earth- 
worm nevertheless responds to light thrown upon its body by moving 
out of the way as rapidly as it can. We know from this experiment 
that the earthworm is sensitive to light and that, therefore, there are 
definite sensory regions, more or less specialized, in its skin by which 
it can receive a stimulus from light. 

It would profit an animal little to be able to receive a stimulus if 
it could not in some way move itself toward or away from such stimu- 
lus. The muscles by which an animal may move out of '^liarm's way or 
toward a food stimulus, and the glands which can secrete substances 
that will repel an enemy, serve such a purpose. 

In order, then, that an animal may profit by the various stimuli it 
encounters, it must have (1), a sensory region or surface of some kind 
which such stimuli may aflfect ; (2), it must have an organ, such as a 
muscle or a gland, which will permit a reaction to the stimuli, (3), it 
must have a conducting mechanism by which the stimulus may be sent 
to the reacting organs. 

The nerve cells become specialized in structure (or in their man- 
ner of connection) in three different ways, namely: (1) they may 
develop sensitivity and form organs of special sense. These nerve cells 
then receive specific stimuli. (2) If the nerve cell develops conductivity, 
it can transmit impulses, such as sensory, to the central nervous system, 
. or motor, from the central nervous system. The conducting parts formed 
by this second group of specialized neurons form nerve tracts. (3) The 
third type of specialization of nerve cells is found in the central nervous 

Nervous System 


system itself. This type forms what are called correlation and associa- 
tion fibres in the sensory field, and coordination fibres in the motor field.. 
In practically all parts of the skin, there are tiny nerve endings, 
commonly called free nerve terminations, by which the individual recog- 

nizes substances when he 

comes in contact with them. 
Then there are certain parts of 
the tips of the fingers where 
definite end organs are found, 
and where the sense of touch 
is quite highly developed. The 
nerve endings in such special 
tactile regions are much more 
complicated than in the simple 
free nerve terminations. Fig- 
ure 485 shows some of the 
various types of these tactile 


Fig. 486. 

A, Labyrinth of human embryo, 30 mm. long. 
B, Section through the cochlea of a guinea pig. a., 
ampullus; ac, anterior canal; c, cochlea; cr., crus; 
de., endolymph duct; Is., spiral ligament; nc, cochlear 
nerve; r., Reissner's or vestibular membrane; s., 
sacculus; se., endolymph sac; sg., spiral ganglion; 
sm,st,sv., scalae media (ductus cochlearis), tympani 
and vestibuli; t, tunnel; u, utriculus; v., vestibular 
nerve. (From Kingsley, A after Streeter and B after 

In our embryological study 
we have already discussed the 
lateral line organs (Figs. 340, 
479) which are in all probabil- 
ity tactile, and probably even 
sound-perceiving organs. In 
the higher vertebrates there are 
three great divisions of the ear, namely, an external, internal, and middle 
ear. Of these three portions, the inner ear is the most primitive. All 
lower vertebrates that develop a definite ear organ at all begin by having 
simply an inner ear (Fig. 19, Vol. I). To this the next succeeding higher 
forms add the middle ear or tympanum, while the highest forms add 
the outer ear. 

The Inner Ear. This consists of a mere area of thickened ectoderm 
on each side of the head between the seventh and ninth cranial nerves. 
A review of the' embryology of the ear will recall the cup-shaped auditory 
vesicle. In the dogfish, the cavity of this remains connected with the 
exterior by a slender tube known as the endolymph duct (Fig. 486). In 
the frog and in higher forms there is no open auditory cup. There are 
two layers of ectoderm, the outer one forming an unbroken sheet across 
the cup. In the dogfish, these endolymph ducts can be seen as external 
portions on the top of the head. 

The distal end of this endolymph duct becomes enlarged, the enlarge- 
ment being called the sacculus endolymphaticus. In the frog and other 
amphibia, the ducts of both sides often unite dorsal to the brain, while 
the other parts branch and extend into the spinal canal in a root-like 


Comparative Anatomy 

manner. In the frog definite diverticula are sent into the so-called cal- 
careous g-lands surrounding the basal portion of the spinal nerves. 

The auditory vesicle constricts into 
two chambers, an upper vestibule or utric- 
ulus and a lower sacculus, connected by 
a narrow sacculo-utricular canal. Three 
outgrowths now take place, one each from 
the outer, posterior, and lateral walls of. 
the utriculus ; the one from the lateral 
wall lies in a horizontal plane, the others^ 
in vertical planes. These outgrowths form 
tubes, and as they are circular in outline, 
they are called the semi-circular canals. 
Some of the sensory epithelium has 
spread into all of these regions, but a defi- 
nite patch of this sensory epithelium can 
be seen in each of the semi-circular canals, 
and it is around this patch that the wall of 
the canal expands to form an ampulla. 
Figure 486 will have to be studied and 
a model of the ear seen or an ear defi- 
nitely worked out in one of the animals 
to make this clear. 

In forms higher than fish and amphi- 
bia, there is a little pocket, called the 
lagena, given ofif from the posterior side 
of the sacculus. Sensory epithelium 
extends into this pocket, and in the higher 
forms the lagena becomes a peculiar struc- 
ture called the cochclear duct. 

The structures of the internal ear, just